[time-nuts] LTE-Lite

Jim Lux jimlux at earthlink.net
Sun Nov 30 17:43:34 EST 2014

On 11/30/14, 1:49 PM, Hal Murray wrote:
> eb4apl at 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?  ...


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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