[sr.org] NAD27 UTM to WSG84 lat/long accuracy

[Christian Gendreau]

Context:
I want to transform a NAD27 UTM Zone 10(EPSG:26710) coordinate into a
WSG84 lat/long coordinate.

The source coordinate is:
nad27 / utm zone 10n 0487871 5456911

Transformations:
-Using GPS: 49.26665, -123.16801 [1]
-Using QGIS: 49.266639175577794, -123.168025173802704 [2]
-Using PostGIS: 49.2668114883735, -123.166713923414 [3]
-Using GeoTools: 49.2665204686265, -123.16771327170319 [4]

[1] This was not taken by me but it seems to match the QGIS converted value
[2] QGIS 2.0.1
[3] PostGIS 2.0.1 Query: SELECT ST_AsText(
ST_Transform(ST_GeomFromText('POINT(0487871 5456911)', 26710),4326))
[4] GeoTools 10 Code snippet:
MathTransform transform = CRS.findMathTransform(sourceCRS, TARGET_CRS);
Geometry targetGeometry = JTS.transform( sourcePoint, transform);

I notice QGIS and GeoTools are using a TOWGS84 parameters in their UTM
coordinate reference system definition.

Questions:
-Is this variation in the accuracy of the transformation expected?
(150 meters offset between GPS and PostGIS)
-Why do parameters for a specific EPSG code differ from one tool to another?

Thank you