[Proj] NAD83(Original) Transformation with Heights

support.mn at elisanet.fi support.mn at elisanet.fi
Fri Feb 14 06:14:13 EST 2014

Rob Skelly [rob at dijital.ca] kirjoitti: 
> Hello List,
> I'm trying to convert some LiDAR data from NAD83(Original), with
> orthometric heights based on CGVD28, to WGS84 with ellipsoidal heights, and
> to NAD83(CSRS) with orthometric heights based on CGVD2013.
> Now, I don't actually have a decent way of verifying my output, other than
> with GPS-H, and the disagreement seems significant. For example, the
> original elevation at 494618.88mE, 6306815.69mN is 469.95m, and I get an
> ellipsoidal height at -111.088356°W, 56.90501°N of 444.52m. GPS-H claims
> that the orthometric height at that position (for HT2_0, the closest
> approximation to CGVD28) should be 470.004m, a difference of 54cm.
> I realize there are a lot of points where this process could be going
> wrong, or that it's not going wrong at all. I wonder if anyone can give me
> any insight or fill in any obvious holes in my reasoning. If I've left out
> any pertinent information, let me know.

ok... "exactly"...

1) have you verified that the 54 cm is not within the limits of the
difference between HT2_0 and CGVD28?? if that is so then there is
no error

2) another source of errors might be the path you go to that elevation..
make sure the selected path (down and up) does not generate that error?

3) make sure that "las2las" and GPS-H do the same assumptions

4) remember that for later proj.4 versions you have to declare both
source and destination datums to make sure that it calculates

5) how large was the horizontal difference?

6) etc etc ..

regards: Janne.

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