Interactive distance measurement accuracy in ArcView
I currently use ArcView 3.2 for basic mapping needs (I know it's archaic) and was wondering what the precision/accuracy of distance measurements made in ArcView are. My current data are in UTM projection (Zone 15N) and I am making measurements more on the scale of kilometers rather than meters. Any help would be appreciated. I searched the ArcView 3.2 help file but could not find any info on how precise/accurate measurements made with the distance measurement tool are. Thanks.
Mapping Center Answer:
Normally we wouldn't answer this question because it was asked specifically about ArcView GIS. However, given the way it was asked, the answer contains some information that may be helpful for ArcGIS users as well.
The answer is mostly a function of your map's scale relative to your computer screen's resolution. that means, the more you zoom in the closer the measurement you derive is to the actual distance. (Note that the measure tool in ArcGIS was vastly improved at version 9.2, and there is a help topic on measuring distances and areas that fully explains what is possible, which includes snapping to geometry in your map)
However, there are two major caveats. First is the projection you are using; not all projections are created equally with respect to linear distance measurement. UTM is just fine, in fact it was intended for just this use on topographic maps. However, in a software environment, be careful about measuring distances across zones. You should also not measure distances that are relatively long compared to the 6 degrees each UTM zone covers. (Your computer screen will inhibit the accuracy of this measurement in ArcView GIS long before the projection does. However, in ArcMap, since you can snap to features, be aware that measuring these relatively long distances will not be as accurate.)
Second is the idea that you're not measuring along the surface of the earth, but a horizontal line (no change in z-value); i.e., it's not like running a distance measuring wheel on the ground. Depending on your terrain, the difference between on-the-ground measurement and horizontal line measurement could be insignificant, however if you have relatively high local relief, horizontal line measurement could underestimate by more than 10%.
Last, in ArcView GIS, the display limits you to 2 decimal places of precision. So, when miles are too coarse, change to feet to get more specific values as you measure. In ArcGIS we show three digits to the right of the decimal point.
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