[Liblas-devel] Precision of reprojection transforms

[Frank Warmerdam]

On 11-04-29 08:35 AM, Edwards, David wrote:
> I am trying to use libLAS to read an LAS file and perform a coordinate
> transformation.  Points in the file are stored in UTM coordinates.  I would
> like to transform them into geocentric coordinates.  As best I can tell, the
> version of libLAS I am using (1.6.1) does not support conversion to
> geocentric coordinates, so my backup plan is to convert to WGS84 geodetic
> coordinates (lat., long. , height above ellipsoid), and then perform the
> geodetic-to-geocentric conversion myself.
>
> The problem is that when I set up a ReprojectionTransform to convert the
> points from UTM to geodetic coordinates, the resulting latitude and
> longitude of each point has nonzero digits only to the hundredth of a
> degree, even though the UTM coordinates of my input points have data to the
> millimeter.  For example, here are the results of two such conversions:
>
> UTM (Easting, Northing) --->  Geodetic (Longitude, Latitude) 831805.86
> 4296378.26 --->  -77.18 38.75 831806.31 4296377.64 --->  -77.18 38.75
>
> Even though the points have different UTM easting and northing coordinates,
> they are transformed to the same latitude and longitude.  Is this limited
> precision inherent in the way libLAS does its coordinate conversions, or am
> I setting up the transform object(s) incorrectly?
>
> I apologize if this is a duplicate post.  I tried to send this yesterday
> before joining the mailing list.

Dave,

Reprojection is generally accurate to a small fraction of a millimeter.
I believe the problem you are encountering is how you are formatting the
coordinates for reporting - not the nature of the reprojection itself.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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