eb4apl at cembreros.jazztel.es
Sun Nov 30 17:50:52 EST 2014
On 30/11/2014 a las 22:49, 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? 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. 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? ...
> 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?
In fact I'm not familiar with USGS topo maps, but here in Spain for the
small scales(Scale 1:25000 and lower) the streets and roads are not wide
enough to be accurately represented, so a symbol is used instead. The
symbol style is selected to mean the type of road and yes, the center of
the parallel lines corresponds to the center of the street. Another
thing is the overall precision, here it is established that the
precision of the paper maps should be equal to the unaided eye
resolution, about 1/4 mm, so you multiply .25 mm times the scale
denominator and you get the precision. Our main national topographic map
is at 1:25000 scale and its precision is about 6.25 meters. For digital
maps the precision is what the map provider says, since a digital map
can be "enlarged" at will. Usually the precision is consistent with the
intended representation scale in the same terms as the paper maps.
Since Google's geographical data is usually obtained from official
sources, the line maps are quite good, but the satellite images usually
are not very well rectified and stitched, unless they are obtained from
similar sources which put a lot of effort on its accuracy and matching
with the maps. This varies a lot depending the region.
The surveyor maps usually agree very well with GPS, in fact they are
based in GPS measured reference points these days.
This comes from my limited experience, the results can be very different
depending the zone and the date.
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