Re: Maps, Signs, and reality
Posted: Wed Sep 26, 2012 7:44 am
Maps are only approximations of terrain. The more you over lay on a map the more chance there is for errors. Google Maps have many errors in their overlay of roads.
Topography is now "drawn" from 30-meter pixel DEM's (Digital Elevation Models). A computer program interpolates between points. There are pretty sophisticated interpolation programs out there that "look" beyond the actual location being evaluated to see what the larger topographical trends are. Nevertheless, you can actually generate different topographical lines depending on which interpolation program is used. The older USGS maps (say 1970's and earlier) are often more accurate because they were actually created by cartographer. That is why some of the newer USGS map contours look "smoother" . On the old maps a cliff could have been actually hand drawn.
All maps have to be geographically "located". Depending on which map projection is used, actual points on the ground do not usually line up perfectly. So if you take a USFS map (for roads) and overlay on a USGS map (for topography) and overlay on satallite images; none of the layers will exactly match up. Some mis-alignments can by significant - say a quarter mile.
And trails are often re-routed and these changes do not show up on a lot of maps. Also, a map cannot show every little bend and switchback in a trail. When I talk of "miles" in my guidebook, I always clearify it as "Map Miles", and comment that "real miles" will be more - often as much as 20-30% more.
Not all GPS maps are equal! Each GPS uses specific maps as their bases. Most of the newer GPS units use the current USGS 7.5 min DEM based maps. However, the trails and roads that are shown in a GPS can come from many different sources of data. Also, YOU have to set your datum and there are two common datums and if the base maps the GPS maker uses does not correct for different datums, the GPS maps can be off.
Topography is now "drawn" from 30-meter pixel DEM's (Digital Elevation Models). A computer program interpolates between points. There are pretty sophisticated interpolation programs out there that "look" beyond the actual location being evaluated to see what the larger topographical trends are. Nevertheless, you can actually generate different topographical lines depending on which interpolation program is used. The older USGS maps (say 1970's and earlier) are often more accurate because they were actually created by cartographer. That is why some of the newer USGS map contours look "smoother" . On the old maps a cliff could have been actually hand drawn.
All maps have to be geographically "located". Depending on which map projection is used, actual points on the ground do not usually line up perfectly. So if you take a USFS map (for roads) and overlay on a USGS map (for topography) and overlay on satallite images; none of the layers will exactly match up. Some mis-alignments can by significant - say a quarter mile.
And trails are often re-routed and these changes do not show up on a lot of maps. Also, a map cannot show every little bend and switchback in a trail. When I talk of "miles" in my guidebook, I always clearify it as "Map Miles", and comment that "real miles" will be more - often as much as 20-30% more.
Not all GPS maps are equal! Each GPS uses specific maps as their bases. Most of the newer GPS units use the current USGS 7.5 min DEM based maps. However, the trails and roads that are shown in a GPS can come from many different sources of data. Also, YOU have to set your datum and there are two common datums and if the base maps the GPS maker uses does not correct for different datums, the GPS maps can be off.