Here is a quick video I did this morning using the very cool tool Gource. The video shows all the commits (1265) that were made between release 1.6 and 1.7. QGIS 1.7 was released on 19th June 2011 but there was still some clean up in the 1.7 branch paste that date into September.

It’s interesting to note the large burst of activity around November.

Last week I found a nice little undocumented feature of QGIS. I plan on writing documentation, so it won’t stay that way for long but I thought I would post about it first and run though it step by step.

This post is going to be a follow up post based on what Tim Sutton did for the same subject back in 2009 at http://linfiniti.com/2009/11/creating-dynamic-forms-a-new-feature-headed-for-qgis-1-4/

For data entry one feature I really like in QGIS is the automatic feature edit forms with support for textboxs, dropdowns and all sorts of other cool Qt controls to make data entry a breeze.

However one thing that people might not be aware of is that you can have a custom forms for data entry. QGIS will take care of setting all the fields and then saving the values back to your layer.

This could be handy if you want to have say a logo, some validation and maybe some text to help the user fill in the form correctly. Or just a custom form layout because you can.

One thing Tim didn’t follow up on was a post about how to add custom Python logic to the form, which I think is the coolest feature of having these custom forms.

So lets get started.

Creating the custom form

This process is pretty much the same as what Tim outlined in his blog post however I’m going to go over it again for completeness.

In order to create the custom form you will need to install Qt Desinger. For windows I haven’t found a way to just install the desinger although if you have QGIS installed it is normally installed with the Qt framework and can be found at C:\OSGeo4w\bin\designer.exe. If you’re on Linux you can run something like

sudo apt-get install qt4-designer

Ohh how I wish windows had a package management system :(

Fire up Qt Desinger and select “Dialog with Buttons Bottom”.

Lets throw on a couple of Labels and a few Line Edits for the data. Now set the form to use a Grid Layout (Right Click on empty space on form->Layout->Layout in Grid).

Now the trick in making a custom form for QGIS is naming the object the same as the field. So I my case I have a road layer with the following fields.

- Segment_ID
- Parcel_ID
- Name
- Alias_Name
- Locality
- Parcel_type

For my custom form I only care about Segment_ID and Name, so my form looks like:

Custom form in Qt Designer

Note that I have set the read only property of the Segment ID line edit to True so that it can’t be edited. I don’t want people messing around with the ID.

As I said above the tick is in the naming so right click on each line edit and select Change objectName, naming each line edit using the same name as the field. For me the first control is called Segment_ID and the other is called Name.

Make sure the objects are the same name as the field.

Save the form into a new folder, I have put mine in C:\Temp\Roads. Jump back into QGIS, load the properties dialog for the layer. Select the General tab and set Edit UI to the new form .ui file.

Setting the edit form UI file.

Save and exit the properties window. Enable the layer for editing (or not) and select an object with the Identify Feature tool.

Woot! Custom edit form in QGIS.

Magic! As I’m in edit mode any changes I make to the Name line edit will be reflected back on the layer (but not the Segment ID as it’s read only). If you are in non-edit mode then you are given the custom form with everything disabled and a cancel button.

With Python validation and custom logic.

Now creating a custom form like above is pretty cool although having some custom Python validation behind it would be even cooler.

What I want to do is add some validation to the Name field so the user can’t enter null road names.

First save your QGIS project (as the Python code runner will look where the project is saved for the Python module). Again I have saved mine in C:\Temp\Roads as Roads.qgs. Now lets make a new python file in your favourite text editor and add the following code.

from PyQt4.QtCore import *
from PyQt4.QtGui import *

nameField = None
myDialog = None

def formOpen(dialog,layerid,featureid):
	global myDialog
	myDialog = dialog
	global nameField
	nameField = dialog.findChild(QLineEdit,"Name")
	buttonBox = dialog.findChild(QDialogButtonBox,"buttonBox")

	# Disconnect the signal that QGIS has wired up for the dialog to the button box.
	buttonBox.accepted.disconnect(myDialog.accept)

	# Wire up our own signals.
	buttonBox.accepted.connect(validate)
	buttonBox.rejected.connect(myDialog.reject)

def validate():
  # Make sure that the name field isn't empty.
	if not nameField.text().length() > 0:
		msgBox = QMessageBox()
		msgBox.setText("Name field can not be null.")
		msgBox.exec_()
	else:
		# Return the form as accpeted to QGIS.
		myDialog.accept()

Wow! What the hell is all that! I’ll step though the code to explain each bit.

Code break down.

First import the modules from Qt and set up a few global variables to hold the dialog and name field.

from PyQt4.QtCore import *
from PyQt4.QtGui import *

nameField = None
myDialog = None

Now we create a method that QGIS will call when it loads the form. This method takes an instance of our custom dialog, the Layer ID, and the Feature ID.

def formOpen(dialog,layerid,featureid):

Then using the findChild method we want to grab the reference to the Name field and the button box. We are also calling buttonBox.accepted.disconnect() to disconnect the slots that QGIS has auto wired up to our button box, we do this so we can hook up our own accepted logic.

After we have disconnected the accepted signal we can wire up our own call to the validate method using buttonBox.accepted.connect(validate).

global myDialog
myDialog = dialog
global nameField
nameField = dialog.findChild(QLineEdit,"Name")
buttonBox = dialog.findChild(QDialogButtonBox,"buttonBox")

# Disconnect the signal that QGIS has wired up for the dialog to the button box.
buttonBox.accepted.disconnect(myDialog.accept)
# Wire up our own signals.
buttonBox.accepted.connect(validate)
buttonBox.rejected.connect(myDialog.reject)

We need a method to validate the logic. This will be called when the signal buttonBox.accepted() is called. The logic in this method should be pretty streight forward. If the Name line edit has a length > 0 then we accept the dialog, if not then we give the user a message and let them fix the mistake.

def validate():
  # Make sure that the name field isn't empty.
	if not nameField.text().length() > 0:
		msgBox = QMessageBox()
		msgBox.setText("Name field can not be null.")
		msgBox.exec_()
	else:
		# Return the form as accpeted to QGIS.
		myDialog.accept()

Almost done!

Now that you have a Python file with the custom validation logic we need to tell QGIS to use this logic for the form. First save the Python file in the same directory as your project. I have called mine C:\Temp\Roads\RoadForm.py.

Back on the General tab in the layer properties we can set the Init function field. We set this to call the module and function we just made. The syntax is {module name}.{function name}. In my case my module (the Python file we made before) is called RoadForm and the function is called formOpen, so it will be RoadForm.formOpen.

Set the Init function field to moduleName.functionName

Save and use the Identify Feature tool to select a feature. You shouldn’t get any errors if everything worked ok. Now delete everything in the Name field and hit Ok.

Validation in action

Sweet! The form can now not be accepted if the name field is null.

And that’s that. Pretty simple but powerful feature once you know how to set it up.

Enjoy!

If you do end up using this custom form with python logic stuff in the real world, leave a comment and maybe a picture. It would be good to see use cases for this cool QGIS feature.

Bonus

Why not add a red highlight to the textbox if something is not valid.

from PyQt4.QtCore import *
from PyQt4.QtGui import *

nameField = None
myDialog = None

def formOpen(dialog,layerid,featureid):
  global myDialog
  myDialog = dialog
  global nameField
  nameField = dialog.findChild(QLineEdit,"Name")
  buttonBox = dialog.findChild(QDialogButtonBox,"buttonBox")

  nameField.textChanged.connect(Name_onTextChanged)
  # Disconnect the signal that QGIS has wired up for the dialog to the button box.
  buttonBox.accepted.disconnect(myDialog.accept)
  # Wire up our own signals.
  buttonBox.accepted.connect(validate)
  buttonBox.rejected.connect(myDialog.reject)

def validate():
  # Make sure that the name field isn't empty.
  if not nameField.text().length() > 0:
    nameField.setStyleSheet("background-color: rgba(255, 107, 107, 150);")
    msgBox = QMessageBox()
    msgBox.setText("Name field can not be null.")
    msgBox.exec_()
  else:
  # Return the form as accpeted to QGIS.
    myDialog.accept()

def Name_onTextChanged(text):
  if not nameField.text().length() > 0:
    nameField.setStyleSheet("background-color: rgba(255, 107, 107, 150);")
  else:
    nameField.setStyleSheet("")

The key part of of this is nameField.textChanged.connect(Name_onTextChanged) and the Name_onTextChanged(text) method. Give it a try, I think it looks quite nice.

Change text background to red on invalid input.

Today, I’ve been experimenting with data from OpenFlights.org. They offer airport, airline and route data for download. The first idea that came to mind was to connect airports on a shared route by lines. This kind of visualization just looks much nicer if the connections are curved instead of simple straight lines.

Luckily, that’s pretty easy to do using PostGIS. After loading airport positions and route data, we can create the connection lines like this (based on [postgis-users] Great circle as a linestring):

UPDATE experimental.airroutes
SET the_geom =
(SELECT ST_Transform(ST_Segmentize(ST_MakeLine(
       ST_Transform(a.the_geom, 953027),
       ST_Transform(b.the_geom, 953027)
     ), 100000 ), 4326 )
 FROM experimental.airports a, experimental.airports b
 WHERE a.id = airroutes.source_id
   AND b.id = airroutes.dest_id
);

The CRS used in the query is not available in PostGIS by default. You can add it like this (source: spatialreference.org):

INSERT into spatial_ref_sys (srid, auth_name, auth_srid, proj4text, srtext) values ( 953027, 'esri', 53027, '+proj=eqdc +lat_0=0 +lon_0=0 +lat_1=60 +lat_2=60 +x_0=0 +y_0=0 +a=6371000 +b=6371000 +units=m +no_defs ', 'PROJCS["Sphere_Equidistant_Conic",GEOGCS["GCS_Sphere",DATUM["Not_specified_based_on_Authalic_Sphere",SPHEROID["Sphere",6371000,0]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Equidistant_Conic"],PARAMETER["False_Easting",0],PARAMETER["False_Northing",0],PARAMETER["Central_Meridian",0],PARAMETER["Standard_Parallel_1",60],PARAMETER["Standard_Parallel_2",60],PARAMETER["Latitude_Of_Origin",0],UNIT["Meter",1],AUTHORITY["EPSG","53027"]]');

This is an example visualization (done in QGIS) showing only flight routes starting from Vienna International Airport:

Flight routes from Vienna International

Connections crossing the date line are currently more problematic. Lines would have to be split, otherwise this is what you’ll get:

Date line trouble

Hopefully this tool can be of some use to people, as I know it has been very helpful to me since I made it.

As I’m a pretty heavy QGIS user now, and my work place still stores most, if not all, of our data MapInfo TAB format, one  friction point for me using QGIS was having to restyle all the MapInfo layers.  If we only had a handful of layer this wouldn’t be such a pain but we have a lot of tables and it would take me months to go though each one manually and style them.

I thought “there has to be some way I can automate this…” and so the MapInfo To QGIS Style Generator (or mapinfoToQgis.py) was born. Knowing that QGIS uses QML (a XML file format) to store it style information, and that MapInfo was able to export a style string for each object, I compared what QGIS generated for its QML using the same symbol I picked in QGIS as I had in MapInfo.   Almost a 1 to 1 conversion! Once I worked out how to convert MapInfo point size  to QGIS symbol size, and MapInfo colour value to RGB it was just a matter of generating a QML with the correct values.

Long story short, after a bit of clean up and writing a user guide I would like release version 0.1 of the MapInfo To QGIS Style Generator for wider testing.

Here is a quick example of the output.

Step 1: Take One MapInfo table.

Step 2: Run it though mapinfoToQgis.py

python mapinfoToQgis.py WaterFittings.Tab WatterFittings.qml -c FittingType --UseMapInfo

Step 3: Load QML file in QGIS

Result from running mapinfoToQGIS.py

Step 4: Get a beer?

If you are using MapInfo Font symbols or normal MapInfo 3.0 everything should come across almost exactly. mapinfoToQgis.py will use the same fonts in QGIS as you did in MapInfo and select to the correct symbol size. Although if you are using custom MapInfo 3.0 symbols you will get the default QGIS black square symbol,you can just change it to something better after loading the QML.

Currently the program only support converting symbols but I plan on adding line and region support sometime in the future.

The program can be found at https://github.com/NathanW2/MapInfo-to-QGIS-style-generator and more detailed instructions and download link can be found at https://github.com/NathanW2/MapInfo-to-QGIS-style-generator/wiki/Using-MapInfo-to-QGIS-style-generator.

Like I said at the start, hopefully other people will find this tool handy as I know I have.  If you do find it handy let me know, I would love to hear peoples feedback.  Also if you find any bugs let me know in the comments or log a issue on https://github.com/NathanW2/MapInfo-to-QGIS-style-generator/issues

Enjoy :)

Note: The following post only applies to QGIS 1.7.  QGIS 1.7.1 (upcoming patch) will not have this issue as the default behavior has changed. 

One thing I noticed when running QGIS 1.7 was my rasters were really slow to open and sometimes froze QGIS.  I thought maybe it was some bad plugin that I had installed, nope; maybe opening the raster of the network drive was causing it to be slow, nope. I tried a bunch of stuff, still nothing.  After a while a few other people posted to the mailing list saying they were having the same issue, turns out the solution was very simple.

In QGIS contrast enhancement for rasters is turned on by default, so each time a raster is opened QGIS had to calculate the stats (max and min for example) for the raster and then scale the contrast.  For a large raster this is pretty heavy and this was the cause of all those problems.

So what’s the solution?

Turns out QGIS will remember the contrast setting for all rasters if you want it to.

To fix the slow opening raster problem:

1. Open a small raster (just any old picture will do)
2. Double click the layer in the layer list
3. Change Contrast Enhancement to No Stretch
   

   Change Contrast Enhancement to No Stretch

4. Hit the little Save icon next to the drop down box
5. Open your big raster.

That setting will now be remembered for each raster that is opened and improve the loading time.

On a similar note.  Tim over at http://linfiniti.com has done some work to improve the raster performance even more with http://linfiniti.com/2011/08/improvements-to-raster-performance-in-qgis-master/ .  The improvements that  Tim has made are available in the nightly build of QGIS via qgis-dev in OSGeo4W, or by building the master line from github.com.

One trend that come up a lot in the poll I recently ran (results will be out soon, just writing a summary blog post) is “Needs more powerful edit tools“.  And I agree.

Coming from a MapInfo background nothing much changes when you move to QGIS, MapInfo had the simple Add Point, Add Region, Add Line kind of tools and then you need to use plugins to do anything a bit more advanced (MapCAD).  QGIS has the start of a kind of MapCAD thing happening although not as complete.

Why not just let people write plugins?

However I think in order to make QGIS a more attractive package to a lot of people it needs to get some built in semi-advanced to advanced editing tools;  getting back to those poll results of “Needs more powerful edit tools”.

I thought to myself “What would I like QGIS to be able to do when it came to editing?” although I came up with a small set my editing needs are only relatively small.  I then thought the best way to find out what people need is to just create a blank canvas for people to throw their ideas around on and then go from there.

Why not just create a ticket?

My idea is to get a larger idea of what people need and want rather than just one-off tickets. Although the ideas will be at the edit tool idea level it will be easier to see how they should all fit together if they are all in one place and editable by other people.  You can then start asking questions like: Do we really need that UI there?  Can we merge these tools?  How should the output be handled?

The overall goal is to have a tight, thought out, group of edit tools rather then someone creating a plugin over here for one thing and some else creating something else over there.

So how do we get this going?

Well I have created a Google Document that anyone can edit and view in order to start brainstorming ideas. The link can be found here.

I have already created an example of two ideas that I would like to see.

So go ahead, throw some ideas up.  I’m interested to see where this can head.

Why use Google Docs and not the QGIS wiki?

Mainly because Google Docs makes it very easy to do frictionless editing of a document together.  No need for user names or passwords or overwriting someone else’s changes (Google Docs is all real time).

If your a QGIS user, and you haven’t done so already, how about consider adding yourself to the QGIS user map at http://planet.qgis.org/community-map/. So far there are 604 user mapped

QGIS User Map

although Australia doesn’t look like it has many users…….yet.

QGIS user map Australia

So if you are a QGIS user (full time or part time) it would be good to see your location on the user map esp if you are in Australia :)

Want to finally try QGIS Server on your Windows system? Thankfully, Till Adams has written a new HowTo for QGIS Server on Windows. Till shows how to install OSGeo4W version and how to get it running.

If you first want to see what QGIS Server and QGIS Webclient can do, check Webgis-Uster homepage by Andreas Neumann.

By default QGIS uses the Qt QPlastiqueStyle for all its windows which makes QGIS render all controls like this:

Default style for QGIS on windows.

However it can be changed in order to make it fit in with system style, as in use Windows style. Running the following commands in the python console:

from PyQt4.QtGui import QApplication
QApplication.setStyle("windowsvista")

will change QGIS to render controls in windows style:

QGIS using windows style.

It’s quite hard to notice many of the differences but just try it you will see them

However there is one limitation. QGIS will not remember that setting as the compiled binary forces QGIS running on Windows into QPlastiqueStyle.

I have started working on a patch to give the user a choice in the options which control rendering style they would like to use. I think choice in these situations is good because I have grown to like the QPlastiqueStyle, mainly because I have used it so much , but other people I know prefer the Windows Vista style controls.

Vienna’s Open Government Data initiative is publishing an increasing amount of Geodata and the best thing is: They’re publishing it via open standardized services! Both WMS and WFS are available and ready to be used in QGIS.

Let’s see how we can use the data available through their WFS using the district information layer “Bezirksgrenzen” as an example. The page lists a GML and a GeoJSON version of WFS. For now, we’ll use GeoJSON.

In QGIS, the layer can be loaded using “Add vector layer” – “Protocol” and inserting the GeoJSON url there. The encoding should be changed to ISO8859-15 to account for “Umlaute”.

The loaded GeoJSON layer "Bezirksgrenzen"

Now, we have geodata. Let’s add some attribute data too! Attribute data is available in CSV format. After downloading e.g. some information on the district population, check the file content and remove excessive header lines so that there is only one header line containing attribute names left. Then, you can load the CSV file into QGIS too (“Add vector layer”).

Last step: Joining geodata and attribute data! Go to the vector layer’s properties – Join tab and add the following join relation:

Joining GeoJSON and attribute layer

Now, the attribute table of the vector layer contains the additional CSV attributes – ready for further analysis. If you want to classify based on numerical CSV attributes, you’ll have to create a .csvt file first otherwise all fields are interpreted as texts.

Works great. Thumbs up for this great initiative!

I have just whipped up a small user poll for QGIS users. The poll is just to give me (and indirectly the team) some idea of people’s opinions about QGIS and what it could do better. The poll is only short (9 questions) and one of the main things is “What could QGIS do better?”.

The poll can be found here: https://spreadsheets.google.com/spreadsheet/viewform?formkey=dDA5TW96Z19aNFU1OVpzSzFyRzFybmc6MQ

All the answers are anonymous so feel free to say what you like about anything. In the end if something isn’t working the way you think it should, write it down. It’s not going to hurt anyone’s feelings :)

If you are a partial QGIS users who mostly uses MapInfo or ESRI stuff I would also like to get your opinion.

Another cool open source project that I have become a part of (as a QGIS packager and tester) is the OSGeo-Live project.  The OSGeo-Live project is a live DVD/USB/Virtual Machine built on xUbuntu(striped down Ubuntu linux) that has a lot of cool open source geo spatial programs all set up and ready to use.

The OSGeo-Live project contains:

• Browser clients
• A small sample of crisis management software
• All the popular database engines (PostGIS, SpaitalLite etc)
• Pretty much all the open source desktop GIS apps (QGIS, uDig etc)
• Open Source GPS navigation apps and globes.
• A collection of handy spatial tools
• A ready to go web services ready to try in your browser or desktop GIS.
• Some sample data to get started with for each project
• And quick starts for each program.

The full list of software contained on the OSGeo-Live project can be found at http://live.osgeo.org/en/overview/overview.html

This is a good project if you want to get into the OSGeo tools and experiment but don’t want to install them on main machine until you know what you need.

As it is a live DVD/USB/Virtual Machine some apps will run slower than what they do on a native install but overall the speed is usable and good enough for testing.

Even better is that it was born in Australia :)

The project is also commercial supported by a Australian company http://lisasoft.com

So give it a try if you are interested in the OSGeo movement, which you should be if you are reading my blog :)

QGIS 1.7 has landed. After some delays due to a major infrastructure overhaul, a new version of QGIS is available for download. For a list of what’s new in 1.7 check the release announcement.

Tonight Tim Sutton officially made the release announcement for QGIS 1.7, so I’m guess I’m free to blog about the newest version now and its cool features.

What are you still doing here? Go and get it! http://www.qgis.org/wiki/Download

I am, as a heavy QGIS user and a guy-who-tries-to-write-features-and-patches-for-the-code, very happy with this release. I know a lot of people have put a lot of hard work and free time into working on features and bug fixes that keep making this free GIS system even better.

Some of the more notable new features in this release are…well there are just way to many for me to list here so go and check out the official list at http://qgis.org/component/content/article/127-qgis-1-7-release.html

The QGIS team has shifted their source control system to using GIT, which I am very happy about as a lot of the guys on the #qgis IRC channel will know :). The bug tracer has also been moved tohttp://hub.qgis.org/projects/quantum-gis.

Since the release of QGIS 1.6 there have been 1199 commits (using git to count: git log –pretty=oneline upstream/release-1_6_0..upstream/release-1_7_0 | wc -l). Not a bad effort if I may so myself.

If you are still reading this, I really hope it’s because you are waiting for QGIS 1.7 to install.

Ever wondered how to create multi-line labels in QGIS? The new labeling engine has a “Multiline labels” option but it’s not so obvious how to create a usable labeling attribute. Here is how it works (credits to @nhopton on QGIS forum):

1. Create a big enough text field (if the data doesn’t contain any yet).
2. In Layer Properties – Fields, chose a “Text edit” edit widget for the label field.
3. Enter the multi-line text into the label field. You can do this using Attribute Table or Feature Form.
   

   A feature form with "Text edit" widget

4. Activate labeling. You’ll have to tick “Multiline labels” option in Layer Labeling Settings – Advanced – Options. That’s it:
   

   Simple multi-line label example

A common use case is the wish to show multiple attribute values in a feature’s label. Using Field Calculator, you can combine them into multi-line labels. All you need is to combine the fields with the || operator and add ‘\n’ (newline) wherever there should be a line break:

Field1 || '\n' || Field2

Populating a multi-line label field using Field Calculator

And finally, the result:

Multi-line labels displaying city name and population

“-T”, this small appendix can be found after many popular GIS-related acronym. But of course, it always means something different. Take for example GIS-T (GIS for Transportation), WFS-T (Transactional WFS) and WMS-T (WMS with time support). The world of acronyms is a fun place!

Let’s see what a WMS-T can do for us. From the WMS standard:

Some geographic information may be available at multiple times (for example, an hourly weather map). A WMS
may announce available times in its service metadata, and the GetMap operation includes a parameter for
requesting a particular time. [...] Depending on the context, time
values may appear as a single value, a list of values, or an interval, …

Currently, only Mapserver supports WMS-T but the Geoserver team is working on it.

Mapserver

MapServer 4.4 and above provides support to interpret the TIME parameter and transform the resulting values into appropriate requests.

Time attributes are specified within the metadata section:

METADATA
"wms_title" "Earthquakes"
"wms_timeextent" "2011-06-01/2011-07-01"
"wms_timeitem" "TIME"
"wms_timedefault" "2011-06-10 12:10:00"
END

Mapserver supports temporal queries for single values, multiple values, single range values or even multiple range values:

...&TIME=2011-06-10&...
...&TIME=2011-06-10, 2004-10-13, 2011-06-19&...
...&TIME=2011-06-10/2011-06-13&...
...&TIME=2011-06-10/2011-06-15, 2011-06-20/2011-06-25&...

Geoserver

GeoSolutions has developed support for TIME and ELEVATION dimensions in WMS.
There are plans to backport this feature to the stable 2.1.x series after the 2.1.1 release.

Configuration of time-enabled layers can be done via the normal user interface:

The following video by GeoSolutions demonstrates the use of Geoserver’s WMS-T:

Both server solutions seem to support only one time attribute per layer. An optional second time attribute would be nice to support datasets with start and end time like Time Manager for QGIS does.

The gis.stackexchange community keeps on growing and it’s questions and answers span a wide spectrum of GIS-related topics. As of today, the page lists 2,700 questions and 6,400 answers.

As was to be expected, ‘arcgis’ (549 times) is the most used tag followed by ‘arcgis-10.0′ ;)

From an open source perspective, PostGIS gets most attention (rank 8), followed closely by OpenLayers (rank 9) and Geoserver (rank 10). QGIS is the highest-ranked desktop GIS (rank 13). Users of other open source desktop GIS like uDig or gvSIG don’t seem to be using this platform yet.

Here’s the full list of open source GIS tags with a count of more than 10:

postgis × 149
openlayers × 137
geoserver × 121
qgis × 97
open-source × 94
grass × 85
gdal × 68
postgresql × 32
pgrouting × 20
geodjango × 14
mapnik × 13

Recently, @simo has posted an elegant solution for defining custom styles for Google Maps layers in OpenLayers on gis.stackexchange. An example with source can be found at http://www.empreinte-urbaine.eu/mapping/styled_gmap.html. The idea seems to be to use a StyledMapType:

The StyledMapType allows you to customize the presentation of the standard Google base maps, changing the visual display of such elements as roads, parks, and built-up areas to reflect a different style than that used in the default map type.

How great would it be if it was possible to define such styles in QGIS OpenLayers plugin too!

Turns out the last blog post I did on this subject contained a few errors, mainly that QGIS wouldn’t render the layer when you opened it.

The answer is so obvious it’s almost embarrassing :)

In order to open and display a SQL Server 2008 layer in QGIS correctly, via OGR, you must have a geometry_columns table in your database with the name, geometry type and srid of the layer. That’s it! Oh look, it was even right in front of me in the OGR code for the mssqlspatial driver.

int OGRMSSQLSpatialTableLayer::FetchSRSId()
{
    CPLODBCStatement oStatement = CPLODBCStatement( poDS->GetSession() );
    oStatement.Appendf( "select srid from geometry_columns "
                    "where f_table_schema = '%s' and f_table_name = '%s'",
                    pszSchemaName, pszTableName );

    if( oStatement.ExecuteSQL() && oStatement.Fetch() )
    {
        if ( oStatement.GetColData( 0 ) )
            nSRSId = atoi( oStatement.GetColData( 0 ) );
    }

    return nSRSId;
}

So the process to open a MS SQL 2008 spatial layer in OGR is as follows.

There are two main tables which tell OGR how to read a layers projection:

• geometry_columns
• spatial_ref_sys

geometry_columns contains the table name and the key for the table spatial_ref_sys which contains the projection string. The projection string is the info that QGIS needs in order to correctly render a layer.

The easiest way to get the correct tables is to let OGR handle it for you via ogr2ogr, then just adding any other tables you may have already in your database to the geometry_columns table.

So to get ogr2ogr to create the right tables for you it’s as simple as running the following command from inside the OSGeo4W shell, changing the connection string part of course:

ogr2ogr -overwrite -f MSSQLSpatial "MSSQL:server=.\MSSQLSERVER2008;database=geodb;trusted_connection=yes" "rivers.tab"

(sample taken from http://www.gdal.org/ogr/drv_mssqlspatial.html)

Uploading even just one table this way will create both tables and fill in the needed info.
The geometry_columns table:

The spatial_ref_sys table:

So if you have already existing tables in your MS SQL 2008 database that were loaded, via say MapInfo’s EasyLoader, you would just upload one table via ogr2ogr to create the two tables needed by QGIS(using OGR) and then add the other tables to the geometry_columns table. If they are all in the same projection than you are in luck as you will only need to upload one in order to get the right strings in the spatial_ref_sys table, if not just upload a small sample for each projection.

Then you can open the table in QGIS using:

uri = "MSSQL:server={serverName};database={databaseName};tables={tableName};trusted_connection=yes"
qgis.utils.iface.addVectorLayer(uri,'{yourLayerNameHere}','ogr')

Tip: In order to test you have correctly set the table in geometry_columns you can run another ogr tool ogrinfo:

ogrinfo -al "MSSQL:server=localhost;database={your database};tables={your table}" -fid 1

If you see a value in Layer SRS WKT: then chances are it’s set right and QGIS should be able to render it, however if you see: Layer SRS WKT:(unknown) Than chances are QGIS will not render it correctly.

Hopefully this help people use MS SQL 2008 Spatial with QGIS, a important step I think in the world of using QGIS on Windows (especially when you don’t have the freedom to run PostGIS:) ).

I might even do a video tutorial when I get some free time after my exams and my wedding.

One of my favorite features of QGIS is the rule based rendering.

QGIS rule based rendering dialog

If you’re using MapInfo think of thematics + queries but on steroids. Rule based rendering allows you to you set, well, rules on what gets rendered and how.  The rules are based on a simple SQL style query language that’s built into QGIS.

Take for example the above screen shot.   The screen shot is from a current project I am doing in QGIS to clean up our current stormwater/drainage layer.  The layer is a in a bit of a mess at the moment so I needed a way to visualize what I have cleaned up and what I haven’t, so enter QGIS rule based styling.

For example: A pipe that has an upstream and downstream invert and is part of the trunk (main) network is then considered valid (for this situation anyway), so I created the following rule:

network_type = 'Trunk' AND Description != 'Drainage Imaginary Pipe' AND (US_Invert > 0 AND DS_Invert > 0)

We also have little connecting pipes that I don’t want to include in valid trunk  as they are only used to connect pits to pipes and are just cosmetic, I have excluded them by adding “Description !=’Drainage Imaginary Pipe’” to the above filter.

Next I wanted to show invalid trunk network pipes (ones without an up or downstream invert), so we just invert the last condition and swap the last AND for a OR:

network_type = 'Trunk' AND Description != 'Drainage Imaginary Pipe' AND (NOT US_Invert > 0 OR NOT DS_Invert > 0)

I also need to show but no highlight the non trunk pipes and the connecting pipes, so I made the next two rules and set their styles to a light gray:

NOT network_type = 'Trunk' AND NOT Description = 'Drainage Imaginary Pipe'

Description = 'Drainage Imaginary Pipe'

Finally I want to show pipe direction on all pipes but not the connecting pipes, again as they are just cosmetic:

Description != 'Drainage Imaginary Pipe'

You will also note in the screenshot above that I have a max zoom scales set on the last three rules, this is because when I zoom out all that info becomes overwhelming at that scale and distracts from showing the invalid parts of the main trunk line.

So after all that, the results:

Map rendered using rules

and if I zoom out pass 5,000:

Map rendered when zoomed out pass 5,000

I think you can see how this rule based rendering could be very powerful, in fact I have about four different rule sets I use with the drainage layer to show different things to different people.

The rules I have shown above are pretty simple, you can go pretty crazy and use them to render OpenStreetMap data: http://www.youtube.com/watch?v=NBBYtH2svw0

The worst part about the rule based rendering is that I have gotten so used to their power that I feel crippled when I go back to MapInfo and try to do styling :)

Happy mapping.

What is your favorite QGIS feature?