GIS Tool: Bayesian Networks

A Bayesian network - it’s complicated…

As discussed in Episode 12 of the podcast, a Bayesian network is a model that brings known factors as well as unknowns and degrees of certainty of your data into a prediction model. It’s pretty complicated (at least for me), but it may be useful to some of you who are more knowledgeable about probability theory!

A Bayesian network model is something mostly used in academia right now (see academic papers and journal articles here, here, and here, and even notes from a Stanford class here and an older tutorial here), but Esri, QGIS and others are incorporating tools to be able to more easily do this type of modeling inside of GIS software. 

We’ve got this!

A few Esri desktop software tools that use this model include Class Probability which is run against a raster image, Empirical Bayesian Kriging which requires a license of Geostatistical Analyst to run, and Maximum Likelihood Classification which requires the Spatial Analyst extension.

QGIS is a free, open source GIS software. There is a plugin called the PMAT (probabilistic map algebra tool) that users can download in order to integrate Bayesian network models with their geospatial data.

Mark Altaweel wrote a much better article over at GIS Lounge, so why not head over and read that for more information!

If I got anything majorly wrong or if you use this type of modeling in your work, please let us know in the comments below!

GIS Tool: GPS Units

Being the Magical Mapper, I’m often asked about GPS units and why I don’t just use the built-in GPS unit in my smartphone to collect GIS data. While consumer GPS technology is getting better every day, most GIS analysis requires a little bit better locational accuracy. Regular GPS units like recreational GPS units or the chips in smartphones can get you within a few meters if conditions are excellent (estimates are 16 ft. under wide-open skies), but you typically don’t know the accuracy of each point captured (so you know what types of mapping or analysis are appropriate to conduct with your collected data). This article gives a good overview of GPS accuracies, and this blog post explains it in a bit more detail.

Basically, this means that if you’re trying to map a location of an outfall that you’ve tested for poop (er… fecal coliform), and want to get back to it (and not mix it up with other ones in the vicinity), you need a mapping-grade GPS unit and shouldn’t trust your phone (unless you want to make awesome drawings to carry with you).

Mapping-grade GPS units can have accuracies ranging from a few centimeters to within a meter of the actual feature you’re collecting and store the accuracies for each point. With new rules being applied and apps being developed, it’s conceivable that I may use a smartphone one day to collect data…

Most GIS Professionals are still using mapping-grade GPS units, like the yellow Trimble units you may have seen (there are many brands out there but this is the one I’m most familiar with). There are a lot of new versions now that are just a little puck sized device that connects to a tablet or phone via Bluetooth. With with apps such as Fulcrum, or Esri’s Collector or Survey123, you can create a much more user-friendly map or form to use with the GPS unit but still get the accuracy you want. If you don’t care about locational accuracy so much, or if you already have your feature mapped but need to add additional information (or return to take another sample), you can still use these apps to collect data.

Tell us more about how you use GPS in your work on our Facebook page.