Peter,
I want to round the coordinates of the vertices of linestrings represented as
WKT, not the projection parameters. My problem is that databases have
different field types for strings depending on their expected length (MS Access
TEXT vs MEMO for example, and I'm still trying to find info on DBF types).
As you've explained below, instead of representing the latitude of a vertex
as 53.146791166875367, it would be ok to instead use 53.1467912 and be within a
centimeter of the original latitude (as long as the projection parameters
remained at their full precision).
Best Regards,
Brent Fraser
Peter J Halls wrote:
Brent,
why would you want to? Maybe you do not appreciate the implications
of so doing? The parameters of which you complain define the ellipsoid,
the shape and measurements of the Earth, to be used for that
projection. Round them and you effectively change the size and shape of
the Earth!
Based on the Clarke Ellipsoid, 1m at the equator is approximately
0.0008833141949 of a degree; round that and you lose the positional
accuracy - and 1m positional accuracy is, frankly, not great. You can
divide that figure by ten if you want to work at .1m positional
accuracy. However, most mapping agencies and surveyors will probably
use 1cm positional accuracy, or one hundreth: 0.000008833141949. There
are a number of other spheroids and ellipsoids in use, as the surface of
our planet is sufficiently irregular to necessitate different
measurements depending upon the location of interest.
The map projection equations are sensitive to this numerical
precision - they have to be - so reducing the numerical precision of the
projection parameters will significantly reduce the precision of the
final result. Indeed, it does not take much rounding to have a dramatic
result, should you seek to align the results with material projected
using the proper parameters.
You can test this yourself be experimenting with some spherical
trigonometry - the mathematics necessary for working with position on a
sphere. You can find the equations for the various projections in J P
Snyder's 'Map Projections: a working manual', which is available online
from USGS. Try out some of the equations by hand, with the full
precision and with your proposed precision for some locations in your
area of interest and I'm sure you will then appreciate the need for the
precision in these parameters.
Best wishes,
Peter
Brent Fraser wrote:
I've been experimenting with v1.6.0 ogr2ogr:
ogr2ogr -f csv test_dir test_in.shp -nln test_out -lco GEOMETRY=AS_WKT
The precision of the coordinates in the WKT seems to be overkill, eg:
"LINESTRING (-115.11433812265155
53.146791166875367,-115.12192424362472 53.147304268559473,
Is there any way to limit the precision?
Thanks!
Brent Fraser
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