[pct-l] gps / gis for pct path?

[AsABat at 4Jeffrey.Net (AsABat)]

I had a lengthy discussion with Jeff Schaffer about this a couple years ago,
and fully agree.

However, GPS and GIS are two entirely different animals. I think most of us
understand GPS (Global Positioning System).

GIS is Geographical Information System. It takes data about an area and
relates it to a map. In many cases, the accuracy of a property line, for
example, is not nearly as important as who owns the property, who owns the
properties are adjacent to it, assessed value, population, water lines, etc.

Often this is done using shape files which contain, well, shapes, or, more
specifically, lines and polygons. A line could represent the PCT. It doesn't
have to fit on the map in the exact lat and lon, but rather on a grid which
is common for several layers, such as the trail, topo info, and roads.

It is not uncommon for GIS maps to be deliberately inaccurate. For example,
if a line represents a road and another line represents a sidewalk next to
the road, if the two lines are exactly where they should be, one line could
be hidden by the other. The tech might then "move" one of the lines slightly
so both are visible on the map. The exact location of the road and the
sidewalk, of course, are still available on the as-built drawings. The GIS
user only cares that there is a road and a sidewalk near a given piece of
property, and how the roads and sidewalk interconnect.

AsABat


> One of the problems of GPS mapping of the PCT is that the topographic maps
> are not accurate. Because of this, the guidebook authors drew the trail
> location in the guidebooks relative to the contour lines, not the correct
> lat/long.
>
> For this reason GPS maps of the trail don't track the lines on the map.
>
> In some areas the contour lines show the geographic features as much as
> 0.5-1.0 miles off the actual lat/long position, so it is a bit
> disconcerting to see your track take you over a peak instead of the pass.