Technoviking said:
True to a point. Roads, treelines and even some waterways alter over time; however, contour lines rarely do.
Agreed, and vital in doing a common sense check I'd say
"Trusting" ones' instruments without a common sense check, usually by using hard copy map, can cause your mileage to unexpectedly vary.
An instrument slaved to a GPS (e.g. Vector 21 with DAGR) can produce elevation errors (false altitude) relative to those contour lines used in the map data base to compute firing solutions, whether air or ground delvered, resulting in the munitions going long or short depending on the error.
This link helps explain one of devices, called PSS SOF, used to mitigate this "false" altitude problem, and how this information can be shared jointly.
http://www.dtic.mil/ndia/2007psa_winter/winnefeld.pdf
Another article on how important that is given the increasing number of precision guided weapons becoming available
http://sill-www.army.mil/firesbulletin/2009/Jan_Feb_2009/Jan_Feb_2009_Pages22_27.pdf
Another deadly error can occur if the users don't do a gross error check and too hurriedly just read the GPS data because its always right.
The typical scenario I'm getting at, is with some GPS that resort back to their own last known location if its goes into a dormant or hibernation stage to conserve power. If that same GPS had been used to produce a target grid earlier, and the user prompts the GPS for that target grid, forgetting that it had gone dormant, they might mistakenly read their own grid as the target location. A quick common sense check against a map, preferably by someone that isn't using the GPS, would eliminate that potential.
Wikipedia's explanation covers a number of errors
http://en.wikipedia.org/wiki/Global_Positioning_System
But one that isn't very clear in there, is the error that can come out of the user trying to assist a system by forcing a grid into their navigation system, possibly because of perceived spoofing or satellite dead zones, or even just as part of rushing initial set up. This possibly could make things worse, if the nav system has a Kalman filter that may use this location as correct, and include any user induced errors as it resolves the systems location when the GPS signal does become available..
To me, the point of all that is I agree with the the mantracker, you really do need to know how to read map, if for no other reason then as a gross error check of your instruments, or for the inevitable time when you don't have them.