Some QGIS Odds and Ends

My colleague Joe Paccione recently finished a QGIS tutorial on working with raster data. My introductory tutorial for the GIS Practicum gives only cursory treatment to rasters, so this project was initially conceived to give people additional opportunities to learn about working with them. It focuses on elevation modeling and uses DEMs and DRGs to introduce tiling and warping, and creating hillshades and contour lines.

topo_contour

The tutorial was written using QGIS 2.0 and was tested with version 2.4; thus it’s readily usable with any 2.x version of QGIS. With the rapid progression of QGIS my introductory tutorial for the workshop is becoming woefully outdated, having been written in version 1.8. It’s going to take me quite a while to update (among other things, the image for every darn button has changed) but I plan to have a new version out sometime in the fall, but probably not at the beginning of semester. Since I have a fair amount of work to do any way, I’m going to rethink all of the content and exercises. Meanwhile, Lex Berman at Harvard has updated his wonderfully clear and concise tutorial to Q version 2.x.

The workshops have been successful for turning people on to open source GIS on my campus, to the point were people are using it and coming back to teach me new things – especially when it comes to uncovering useful plugins:

  • I had a student who needed to geocode a bunch of addresses, but since many of them were international I couldn’t turn to my usual geocoding service at Texas A & M. While I’ve used the MMQGIS plugin for quite a while (it has an abundance of useful tools), I NEVER NOTICED that it includes a geocoding option that acts as a GUI for accessing both the Google and Open Streetmap API for geocoding. He discovered it, and it turned out quite well for him.
  • I was helping a prof who was working with a large point file of street signs, and we discovered a handy plugin called Points2One that allowed us to take those points and turn them into lines based on an attribute the points held in common. In this case every sign on a city block shared a common id that allowed us to create lines representing each side of the street on each block.
  • After doing some intersect and difference geoprocessing with shapefiles I was ending up with some dodgy results – orphaned nodes and lines that had duplicate attributes with good polygons. If I was in a database, an easy trick to find these duplicates would be to run a select query where you group by ID and count them, and anything with a count more than two are duplicates – but this was a shapefile. Luckily there’s a handy plugin called Group Stats that lets you create pivot tables, so I used that to do a summary count and identified the culprits. The plugin allowed me to select all the features that matched my criteria of having an id count of 2 or more, so I could eyeball them in the map view and the attribute table. I calculated the geometry for all the features and sorted the selected ones, revealing that all the duplicates had infinitesimally small areas. Then it was a simple matter of select and delete.
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