Hi there,
I am surveying in Manitoba, CAN. UTM 14 North. I surveyed a creek a couple weeks ago, using my own BM that I corrected on CSRS PPP. I cannot find original BM information. On CSRS PPP I can use CGVD2013 or CGVD28(HT2_0), I used CGVD28 in this case. I brought my survey into Leica Infinity and applied the corrections to my two benchmarks, and converted the survey to feet. I need to compare my elevations to an original survey taken in 1965 in DATUM: GEODETIC in feet. When I convert everything in the way I have explained, i get elevations around 260.000 m or 875.00 - 876.00 ft. The elevations on the original survey are around 876.00... in theory my elevations should be higher as the drain has collected sediment, but they are around 2 ft lower.
I was told to try switching from NAVD88 (which I can get my benchmarks in using CSRS PP) to NGVD29, but I am not familiar with NGVD29, is this a United States thing? How different is it from CGVD28? Would It work for my area? If so, can you suggest some ways I could convert my benchmarks into NGVD29? I tried the https://www.ngs.noaa.gov/NCAT/ website but it doesn't seem to want to work for.
Or maybe there's something else I'm doing wrong, or some other trick to this? Or maybe I'm not doing anything wrong and this difference of 2 feet is just due to accuracy changes from 1965 to now.
Any suggestions would be greatly appreciated, thank you!
As an example, I have a corrected BM in UTM 14 North, CGVD28, with the following data:
Northing: 5635603.602 m
Easting: 505547.122 m
Elev (orthometric): 267.447 m
It is really unlikely that NGVD29 is your answer. It is, indeed, a "United States thing". I hope someone with local knowledge will chime in. NAVD88 was a cooperative US/Canada datum, and Canada had to have something before that, but NGVD29 was US only.
That said, the difference between NGVD29 (which is based on many tide gauges located on both coasts) and NAVD88 (which is based on a single selected tide gauge) is relatively small (about 1/2 foot) in the great plains states of of the US. It would be rational to expect that to also be the case with the pre '88 datum in the Canadian Shield.
Plus, 2 feet in the pre-GPS era wasn't all that much. It may be nothing more than a survey precisions issue.
I speculate that the CGVD28(HT2.0) takes GNSS-derived ellipsoid heights applies a gravimetric geoid model then another model to account for the difference between the GPS-derived heights and the CGVD28 vertical datum. This is what the NGS tool does.
Because Canada has its own system, the US tool is designed not to work within Canada except perhaps in border zones.
CGVD28 and the US NGVD29 were tied to tide gauges that were considered at the time to represent a single geopotential surface (they do not).
There are many resources on both official US and Canadian sites discussing the systems and their differences. I show below a graphic depicting the differences along the border between the Canadian heights.
I also add links pertinent to the issue. Figure 8 above is copied from the fourth link.
Vertical datums USA and Canada
https://www.ngs.noaa.gov/datums/vertical/index.shtml
https://www.ngs.noaa.gov/datums/vertical/national-geodetic-vertical-datum-1929.shtml
https://www.ngs.noaa.gov/datums/vertical/north-american-vertical-datum-1988.shtml
NGVD29+2.4'=NAVD88 here,,,, more or less.
So a 2' shift shouldn't be too surprising in Canada.
The interesting thing is that the new update for 2022 is going to be NAVD88-2' or almost back to NGVD29.
Go figure.
In my area ngvd 29 to 88 is 1 ft depending closer to coast is less etc. navd 88 held fathers point if I remember correctly and of course ngvd29 many tide gauges. Which is why the tilt on usa It would seem that Canada would have a way to do the shift from 88 to your datum since it was a joint effort and holding fathers point was part of that decision. I hope you can get it figured out for sure.
