I have to questions concerning measurement.
I was wondering about this statement:
"You want to make accurate measurements from your map and be sure that the spatial analysis options you use in ArcMap calculate distance correctly. Latitude-longitude is a good system for storing spatial data, but not as good for viewing, querying, or analyzing maps. Degrees of latitude and longitude are not consistent units of measure for area, shape, distance, and direction. "
As I understand this that mean that I can not use a geographic coordinate system to measure correct distance. However when choosing Geographic coordinate system -> World -> WGS 1984 I get the distance between London and Los Angeles to be around 8800 km, which is correct according to my GIS textbook. I also tried measuring a street in Norway to look how it turned out in a small scale. This was also the approximately correct measurement.
However using different projected coordinate systems (e.g. Mercator) I got completely wrong distances.
I dont really understand this? I would guess all projected coordinate systems would have errors when measuring large scale distance since it is a flat projection? Shouldn't geographical coordinate systems be better for measuring distance as this is based on a spherical world?
And another question:
If I want to make polygons of an area in one UTM zone and compare the size of it visually by moving it into another polygon in another zone how do I do this without getting error? If I draw a polygon in one UTM zone (lets say UTM zone 45) using UTM 45 as the projection and move it to the reference zone (utm zone 29) will I not get error when changing the utm zone to 29? How do I best solve this issue? I tried to do it with a polygon in zone 33 and measured it. Then I moved it to 29 and then I got a measurement error compared to when I measured the polygon in zone 33. Dont know if this is even possible to solve?