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Using Custom Projections in Calculation Fields

October 26, 2015

Earlier this year we launched calculation fields, and we’re constantly impressed with the ways our customers are using them to add value to their field surveys. I want to demo a neat technique you can use to embed proj4js into a calculation field to gain access to custom GIS projections.

First, I’ll just show you the result:


The idea in the demo is pretty simple. The app uses the LATITUDE() and LONGITUDE() functions to display and store the record location using different projections in separate calculation fields. In the demo app, the projection can also be changed on the fly using a choice field. Here is a table of the projections used in the demo app:


EPSG CodeProj.4 TextEPSG:4326 WGS 84+proj=longlat +datum=WGS84 +no_defsEPSG:3086 NAD83 / Florida GDL Albers+proj=aea +lat_1=24 +lat_2=31.5 +lat_0=24 +lon_0=-84 +x_0=400000 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defsEPSG:3857 Spherical Mercator+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defsEPSG:3031 Antarctic Polar Stereographic (just for fun)+proj=stere +lat_0=-90 +lat_ts=-71 +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defsEPSG:26917 UTM zone 17N+proj=utm +zone=17 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

Including the JavaScript library

To access the external proj4js library in multiple calculation fields, I’ve added a calculation field to the form that just includes the proj4js library. Bear with me here because this is a bit of a “workaround” to attach some code to use in multiple calculation fields. One important thing to remember with calculation fields is that everything in calculation field expressions is “just Javascript”. On Android, expressions are evaluated using an embedded version of the V8 JavaScript Engine and on iOS Fulcrum uses JavaScriptCore. These are the same JavaScript engines Chrome and Safari use. So most pure JavaScript libraries will work, but might require some tweaks depending on how the library is implemented.

Here is the source of the “Proj4js” calculation field that exports a proj4 function we can use in other calculation fields.

Try it Yourself

Here is a link to the demo app in the app gallery. You can add it to your account to try it out yourself.

You can also use this same technique to add other Javascript libraries to Fulcrum calculation fields to do interesting things with your data. Check back for more posts on how to do neat tricks with calculation fields.