I was wondering if someone could shine some light on an issue I am having with ARC. Yesterday I produced some maps of the North Sea area but depending on which Map Coordinate System I used, I had differnt results. The first Map I had produced was with a Coordinate System of WGS 84 Web Mercator. This Map was showing a distance of approx 1100km from Scotland to Norway. The second map I produced had a Coordinate system of Nad 83, UTM z31. This Map had a distance of approx 550km from scotland to Norway, which appears to be correct.
So the question remains as to why the huge difference. i'm thinking its related to the broad use of WGS84 in the first map and Not zoning in on a particlar UTM Zone? i.e, if I had used WGS84 Z31, then the map would have been correct. If someone could help explain this issue to me it would be greatly appreciated.
If you were using the Measure Tool in ArcMap, it may be defaulting to the projected / Cartesian distance. That is, the distance in the projected coordinate system. Usually if you switch the tool to use the geodesic distance, you'll get a closer-to-ground measurement because the geodesic distance is based on the shortest path on the spheroid (ellipsoid) that's being used.
Because, at best, only some distances are maintained in any map projection, measuring the projected coordinate system distance can give some very distorted results. In your particular case, Mercator is definitely not designed to minimize distance distortion, particular at higher latitudes. Meridians (longitude lines) don't converge to the poles in Mercator so distance measurements will get worse and worse as you go North or South away from the Equator. Caveat: Esri's implementation of Mercator does include a standard parallel parameter. Set that to the area of interest and distance measurements will improve, but still won't be very accurate.
UTM uses transverse Mercator which does have the meridians converge at the poles, and is designed for larger scale data. That's why the results are better than Mercator.
Thanks for your help on this matter. In the future, what Coordinate system should I have used in this regions (North Sea/ Norway) to allow for the best represenation of the area? I was thinking WGS 84 Z31 was indead the right one?