On 11/30/14, 1:49 PM, Hal Murray wrote:
eb4...@cembreros.jazztel.es said:
Do not trust Google Earth data for any precision work. The mentioned six
feet are probably due to the geographical data, not to the precission of
your GPS unit. If you look for image seams you can verify the kind of
errors involved.
How good are USGS topo maps for this sort of thing?
USGS National Map Accuracy Standards are 1/50th of an inch at map scale
(essentially, the width of a pencil line). That is, things on the map
are within 1/50th of an inch where they actually are. So, on a
1:250,000 map, one can expect 100 meter errors. On a 1:24,000 map, 10
meter errors, etc.
Most streets are shown
as a pair of parallel lines, but the separation of the lines doesn't match
the actual width of most streets.
That's the symbology: that is, you're seeing a map symbol for a street
of a particular class, not the actual dimensions of the street.
Does the center of that pair on the paper
correspond to the center of the road? Can I use the intersection of a pair
of streets as a reference point? ...
No.
What you could use is the center point for Bench Marks (BM) on the map,
variously represented as crosses or triangles. And, of course, they're
only accurate to 0.02 inches on the map (0.0508 cm).
In practice, most USGS maps are somewhat better than this, assuming you
allow for things like changes since the map revision date. My house is
moving at roughly 1-2cm/year due to tectonic motion, and, so, a map that
was revised in 1980 will be some 30-60cm in error. Since a 1:24,000 map
is the standard 7.5 minute quad, there can be 12 meter uncertainty, and
the map doesn't need to be updated.
ANd then we get into map datums. Is your map NAD27 or WGS84?
How about equivalent maps for other countries?
How well do typical benchmarks agree with GPS?
Are the surveyors maps used for deeds useful in this context?
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