I'm using ArcGIS 10.2. I have some point data (sampling points) and I want to know how far each of these is from the nearest protected area (polygon data). I have tried doing this with both the Join function as well as the near function, it gives me values but they are in decimal degrees, I want it in metres. I have tried changing the projection which does not help, the data is from Namibia so I tried projections: Africa equidistant and WGS 1984 Zone 33S. Any suggestions would be great

  • 1
    How did you change projection? On your dataframe or on the data itself? – Martin Dec 8 '16 at 8:06
  • I used the define projection function – Christie Craig Dec 8 '16 at 8:07
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    That doesn't change the projection, only applies another set of parameters to your current coordinates. You should use Project. – Martin Dec 8 '16 at 8:09
  • I projected the geographic to WGS 1984 and projected to africa equidistant, however I still seem to be getting the distance in decimal degrees, do you know what else could be the problem? – Christie Craig Dec 8 '16 at 8:14

You ran into a very common problem. First of all, I recommend this article on project and define projection. Also this question might help you to understand the problem. Whenever some of my colleagues have this problem, I explain it to them as follows:

Think of your data as a person, i.e. "Jane", holding up her name on a sheet of paper marked "Jane". The sheet of paper represents the defintion (of the projection). What you did is changing her paper to "Emma". However, Jane is still Jane (since you didn't really project her), but everyone thinks she's Emma. In order to really change Jane to Emma, you need to project her, however this only works if you give her her proper name tag first. Projecting Jane with the Emma name tag results in a horrible mess.

Also, you need to select a projected coordinate system in order to obtain a 2D-grid that you can measure in metres or any other length measurement. Geographic coordinate systems are always in an angular unit. Read about the differences here.

So, you need to preceed as follows:

  1. Give back the proper definition to your original data
  2. Open the project tool from your data management toolbox and select the desired projected coordinate system (i.e. projected coordinate systems --> UTM --> southern hemisphere --> 33S)

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