QGIS Workshop on 1th July, Maynooth

Due to high demand, the All-Island Research Observatory (AIRO) will be holding an another ‘Data for Decisions’ GIS Workshop this summer at NUI Maynooth, before planned autumn workshops.

“The ‘Data for Decisions’ GIS Workshop is aimed at planners, policy and decision makers, local partnerships and local authorities researchers and private consultants who are interested in bringing data to the core of their decisions. The workshop is designed around the free and open source Quantum GIS (QGIS) software package and is pitched at those with minimum or no GIS experience.”

The day long workshop will introduce beginners to QGIS, with an emphasis on manipulating and analysing ‘socio-demographic’ datasets using the free and opensource software. The workshop is accredited by the Irish Planing Institute and members will gain 7 IPI CPD credits. More information can be found on the AIRO website:

AIRO ‘Data for Decisions’ – Intro to GIS Workshop 1st of July 2014

How to Georeference a map in QGIS

I was recently asked how to Georeference a map in QGIS, this a common and important task. Raster scans of paper maps and some satellite images are in pixel coordinates, they lack location information (no Longitude and Latitude values). In this case, QGIS will assume a single pixel is 1 metre square and it will place the Scanned map at 0o North, 0o West in the Atlantic Ocean south of Ghana. Not cool.

To prevent this we need to Georeference the map, change pixels coordinates into corresponding geographical values. This is done by clicking a visible landmark on the Scanned map and the exact same landmark on a corresponding georeferenced map, the GIS will attach geographical values to the Scanned map (see this interesting article about artificial landmarks used to Calibrate drones and satellites).

Georeferencing maps in QGIS is straightforward using the excellent OpenLayers Plugin. It allows you to add Google, Bing, OpenStreetMap and aerial photos to QGIS. We can take the geographical values from these maps and add them to our Scanned map. Here’s a step by step guide to achieve this:

1. Go to Plugins Menu – Manage and Install Plugins – Type OpenLayers Plugin – Install Plugin (you might have this plugin already).

2. Go to Plugins Menu again – Go to OpenLayers Plugin – Select “Add Google Physical Layer, Google Satellite Layer or Bing Aerial Layer etc.”… add whatever you like to the QGIS map canvas.

3. Move the map with the Hand tool, use the Magnifying Glass and/or the mouse wheel to Zoom to the same area as your Scanned map.

4. Go to Project Menu – Project Properties – Click CRS – Check the box “Enable ‘on the fly’ CRS transformation – type 3857 into the Filter box – Click WGS 84 / Pseudo Mercator – and hit OK.

5. Go to Raster – Georeferencer – Click “open raster” on the far left (left of the green triangle) – open your scanned map in the Georeferencer window.

6. The next stage is a bit tricky – you will need to find landmarks on your Scanned map that precisely coincide with locations on the Google / Bing / Street / Satellite Map in QGIS.

Tip – You might like to place QGIS and the Georeferencer Window side by side, so you can see e.g. the Scanned map next to a Google or Bing map. Good landmarks include bends in rivers, river confluence, road intersections, the corners of large buildings, rock outcrops, field boundaries and corners and small islands. Don’t click the tops of tall buildings, antennae or trees – locations are measured at ground / street level.

7. Zoom into your Scanned map in the Georeferencer window and the same are in the QGIS window – click a landmark in the Georeference window (using the yellow add point) – a dialogue pops up asking you to enter the coordinates – since you don’t know this value – you then choose click “from map canvas” – the Georeference Widow disappears – Now click the exact same landmark in the QGIS window using the cross hair – the map coordinates are added to the Scanned map the Georeferencer window.

Tip – it’s important to be as accurate as possible, so the final Georeferenced map will coincide with Google, Bing etc. These maps appear to be accurate to a few metres, I measuerd this using GPS. If you need better accuracy, you will need professional survey data and ground control points (bench marks).

8. Super Important – after adding all your Control Points – Save Them – if anything goes wrong you wont have to start from the beginning.

9. After adding about 8 to 12 points, which you are satisfied are in the same place on the Scanned map and the Google / Bing map in QGIS – hit the Green Triangle – the Transformation Settings dialogue will appear

10. Select Transformation Type – the selections available at the top of the drop down menu are e.g. Linear, Helmert – While faster they may give less accurate results – I usually choose Polynomial 2, 3 or thin plate spline – For Sampling – Choose Lanczos (smoother better results) – Choose the path of output Raster (where to save the georeferenced map) – The georeferenced map will be saved as a GeoTiff

11 – You must use the same CRS as the map canvas (see step 4).  Check Target CRS – type 3857 – add WGS 84 / Pseudo Mercator (if you later need to change the CRS of a Raster it is necessary to use the Raster – Reprojection – Warp command).

12. Click the box “Load in QGIS when done” – then Finally click OK!!!

13. Your computer may take a few minutes – it will generate and save a Georeferenced Geotiff and add it to the Map Canvas.

14. If all goes well, the nice Georeferenced Map will appear in the QGIS Window and it will be perfectly aligned above the Google / Bing map OpenStreetMap below.

15. If there are any errors – the map will usually look seriously distorted – common errors involve mixing up a control point.

16. Don’t worry, mixing up a control point is a common mistake – if one of your control points is wrong – find that point and delete it – go to step 7 and repeat for that point (hopefully it’s just one point).

Congratulations that’s it!

Here’s a video demonstrating Georeferencing in QGIS:

Orfeo Toolbox – Satellite Remote Sensing for QGIS

OTB1

Orfeo Toolbox 4.0 (OTB) is now available for QGIS on Ubuntu 13.10 and 14.04 via the UbuntuGIS Unstable repository (it was previously available for earlier versions of Ubuntu and separately MS Windows via OSGeo4W). OTB, which can be accessed via the Processing Toolbox, provides QGIS with advanced satellite remote sensing capabilities, the sophistication of which maybe best appreciated upon thumbing OTB’s rather daunting 732 page manual.

OTB is a library of image processing algorithms based on the medical image processing library ITK v4, adapted for analysis of high resolution satellite imagery. Its algorithms are tuned to cope with huge images, that normally require supercomputer facilities, by using streaming and multithreaded processing.

The OTB project was initiated the French Space Agency (CNES) in 2006 with the objective to provide a suit of opensource tools for the analysis of images produced by the Orfeo constellation of satellites; Pléiades and Cosmo-Skymed. The motto of OTB is “Orfeo Toolbox is not a Black Box”, its algorithms are openly documented and its source code can edited by end users under the GNU compatible French CeCILL opensource license agreement.

The capabilities OTB have greatly increased since its initial release 8 years ago, especially with the release of version 4.0 in Feb 2014. It can now handle the optical sensors of SPOT, QuickBird, WorldView, Landsat and Ikonos, various multispectral sensors (e.g. Hyperion) and synthetic aperture radar imagery of TerraSarX, ERS and Palasar amongst others.

OTB’s sister application Monteverdi 2 can be installed as well. Monteverdi is a GUI batch processor for OTB that allows building complex processing chains by selecting algorithms from a set of simple drop down menus. As for future developments, the following is a demo of Orfeo Toolbox 4.0 controlled via Monteverdi 2.0, it uses the forthcoming OpenGL accelerated ICE API:

Reference:

Inglada, J. & Christophe, E. 2009. The Orfeo Toolbox Remote Sensing Image Processing Software. In: IGARSS (4). 733–736.

QGIS Podcast episode 7 now online

In the latest QGIS Podcast, Tim Sutton interviews Martin Dobias, a core developer of QGIS. Martin discusses recent developments in the QGIS project such as multithreaded rendering, python bindings and improved symbology. Multithreaded rendering, due out with QGIS 2.4, releases the full potential of modern multicore CPUs, resulting in improved responsiveness and a substantial speed-up in map rendering speed.

Podcast episode 7