I've been told that UTM is better for calculating distances on a map, however I can't see the reason for that, and I've got a lot of data that is not in UTM, so it would be good to know that I don't really have to worry about that. Is UTM more accurate than Degrees or something like that?
closed as too broad by Dan C, aldo_tapia, whyzar, Vince, PolyGeo♦ Nov 3 '17 at 23:58
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If your work is at the equator and you're working in a projection that has little distortion at the equator, you can do that calculation on the fly. If you have data that spans the globe and want it to be as accurate in Canada as it is in the Bahamas, converting to UTM before doing the calculation is most accurate and depending on your accuracy requirements, very necessary. This is, of course, not true if your geometry objects are large enough to span multiple UTM zones as their distortion is worst at the edges and beyond.
Every single projection has distortion. Using a local UTM zone ensures that you have minimal distortion on the shapes that fall within a given UTM zone. UTM zones are still using degrees but the distoriton within a given UTM zone is less than if you use a world wide projection.
Degrees do not simply translate into Kilometers/Miles. A degree of longtitude at equator is approximately 111 km, while at the poles it gets down to 0 km. Thus, if you get distances in degrees there is no simple way to translate it to kilometers without error. If you need to measure distances, use projected coordinate system. As you mentioned in the OP, UTM, or any projected coord. system with good local fit will yield more accurate result. You can read more about it here.
In almost any GIS (such as ArcMap or QGIS), you'll be able to calculate distance in almost any units you want, with data loaded in with either a geographic (lat/long) or projected (feet, meters, etc.) coordinate system. The reason is that these programs use "on the fly" projection, so you can set the Data Frame (where the layers are loaded) to whatever coordinate system you want, and the layers will be projected correctly when they are loaded and rendered in the data frame. In that way, the software can make measurements and calculations for differently projected layers at the same time.