{"id":4901,"date":"2025-07-17T00:41:21","date_gmt":"2025-07-17T00:41:21","guid":{"rendered":"https:\/\/blogs.lincoln.ac.nz\/gis\/?p=4901"},"modified":"2025-07-17T00:41:22","modified_gmt":"2025-07-17T00:41:22","slug":"degrees-of-distance","status":"publish","type":"post","link":"https:\/\/blogs.lincoln.ac.nz\/gis\/degrees-of-distance\/","title":{"rendered":"Degrees of Distance"},"content":{"rendered":"\n

When measuring distance it pays to be aware of which coordinate system your data are using.<\/em><\/p>\n\n\n\n

Hot off a discussion of coordinate systems and map projections in the GIS courses, I was reminded of something that threw me for a loop with some of the global scale analysis we’ve been doing lately. This one was around modelling E.coli distributions based on measured values and then extending these as predictions globally. We had a point layer of waterwater treatment plant locations and wanted to use the distance to the nearest river reach as a parameter in the model. The plant locations came from HydroWASTE <\/a>while the river network came from HydroRIVERS<\/a> (both available on the J: drive). I’d love to show you the whole dataset but at the global scale there are too many points to see any detail, so here’s a zoomed in version from our neck of the global woods:<\/p>\n\n\n\n

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(I used Graduated Symbols to symbolise HydroRIVERS using the stream order<\/a> attribute.)<\/p>\n\n\n\n

As noted above, we’re interested in using the distance from each plant to the nearest river reach so the Near <\/a>tool is a great option here. This will add two new attributes to the waste water plant attribute table: NEAR_FID (the identifier for the nearest river feature) and NEAR_DIST (the calculated distance). Pretty straightforward – here’s how I set up the tool:<\/p>\n\n\n\n

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and here’s the resulting table:<\/p>\n\n\n\n

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Wait – what’s going on here? Look at those NEAR_DIST values – none of them is greater than 1 which can’t be the case. Looking more closely at one of those records, we can see that they’re not nearly as close as the values say they are:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

The table tells me that the Methven plant is about 6 mm away from the river, which simply can’t be right.<\/p>\n\n\n\n

Interpreting that value as 6 mm would be correct IF<\/em><\/strong> the distance units were in metres. But they’re not, and that’s the problem here. If we look more closely at the data, we can see that the layer’s coordinate system is WGS 1984:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

WGS84<\/a> is the World Geodetic System (1984). This is the coordinate system that GPS system uses. The above image also shows us the units for this coordinate system: degrees. Recall that WGS84 is a Geographic Coordinate System, meaning it uses latitude and longitude to specify 3D locations on the surface of the earth. The value we see in the NEAR_DIST field is the angle <\/em>between the points and the river features.<\/p>\n\n\n\n

So what next? I think I’ve got a few options:<\/p>\n\n\n\n