[Qgis-developer] inaccuracy with NAD27 UTM zone 10 projection

[Alex Mandel]

On 04/01/2010 12:45 PM, Tara Athan wrote:
> I have two layers I am trying to overlay. One is derived from the USGS
> NED, downloaded from the website with no reprojection - it's in NAD27
> UTM zone 10 (EPSG 26710). The other is a parcel layer that was
> originally in state plane NAD83 California zone 2 (ft) (EPSG: 2226). I
> used QGIS 1.4.0 to reproject the vector layer into EPSG 26710, but when
> I display them together on a map there is an offset of the parcels about
> 150 m westward. I have checked the accuracy of the original data by
> reprojecting the vector layer in another GIS (Idrisi) and in that case
> there is no offset, the two layers match up well. So I have a workaround
> for my immediate needs, but I thought you might like to know about this
> issue.  Would anyone like to check to see if they can reproduce it, or
> should I go ahead and file a bug report?
> 
> Tara
> 
Hmm, 150m westward sounds suspiciously like the datum conversion from
NAD27 to NAD83 isn't being handled correctly.

What happens if you actually reproject the dataset to a new file and
load them?

Also there are few really similar EPSG codes for state plan (though it
shouldn't have this big of an effect) check
http://spatialreference.org/ref/esri/102642/
http://spatialreference.org/ref/esri/102242/

Thanks,
Alex