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Author Topic: What units are exported point clouds in for NAD27?  (Read 5457 times)

nickponline

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What units are exported point clouds in for NAD27?
« on: September 22, 2015, 11:30:24 PM »
I have a point cloud in PS and have exported the point in both WGS84 and NAD27 Zone II. The WGS data looks like this, with elevation in meters:

-121.330272 38.547287 5.661467
-121.330272 38.547287 5.646508
-121.330272 38.547287 5.661565
-121.330272 38.547287 5.657426

where as the NAD27 Zone II data looks like this:

2191840.924977 321431.770306 44.012074
2191840.804111 321431.731749 43.962994
2191840.798730 321431.632417 44.012394
2191840.725906 321431.877987 43.998812
2191840.639473 321431.718561 44.001784

I don't understand why the elevation is different and what units it is meant to be in, doesn't seem to be in either meters or feet. Something else is going on?

nickponline

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Re: What units are exported point clouds in for NAD27?
« Reply #1 on: September 22, 2015, 11:48:02 PM »
Also the original values ellipsoidal heights?  What vertical coordinate system are they in?

Thanks so much for your help.

Paulo

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Re: What units are exported point clouds in for NAD27?
« Reply #2 on: September 23, 2015, 07:44:45 PM »
Hello,

the Nad 27/ California Zone II coordinates have units as US Foot instead of meters so probably that is the reason for big diffrence in Z... here is the Prj file for such projection:

PROJCS["NAD27 / California zone II",GEOGCS["GCS_North_American_1927",DATUM["D_North_American_1927",SPHEROID["Clarke_1866",6378206.4,294.9786982138982]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Lambert_Conformal_Conic"],PARAMETER["standard_parallel_1",39.83333333333334],PARAMETER["standard_parallel_2",38.33333333333334],PARAMETER["latitude_of_origin",37.66666666666666],PARAMETER["central_meridian",-122],PARAMETER["false_easting",2000000],PARAMETER["false_northing",0],UNIT["Foot_US",0.30480060960121924]]
Best Regards,
Paul Pelletier,
Surveyor

nickponline

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Re: What units are exported point clouds in for NAD27?
« Reply #3 on: September 23, 2015, 07:51:24 PM »
Really? But look at the first line for example 5.661467 meters is not 44.012074 feet?

Paulo

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Re: What units are exported point clouds in for NAD27?
« Reply #4 on: September 23, 2015, 08:25:27 PM »
Nickponline,

you are right. This discrepancy can come from the datum transformation from WGS84 to NAD27.

In WGS84, elevations are relative to WGS84 elipsoid (geocentric or centered to Earth's center of mass) while in NAD27 they are relative to regional (non geocentric) Clarke1866 elipsoid. The shift between 2 elipsoids can reflect in Z value changes...
Best Regards,
Paul Pelletier,
Surveyor

nickponline

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Re: What units are exported point clouds in for NAD27?
« Reply #5 on: September 23, 2015, 08:28:51 PM »
That's interesting is there anyway to compensate for this because pyproj doesn't seem to take it into account. If I try and manually transform the first point the elevation doesn't change.

import pyproj

p1 = pyproj.Proj("+init=EPSG:4326")
p2 = pyproj.Proj("+init=EPSG:26742")

a = (-121.330272, 38.547287, 5.661467)
b = (2191840.924977, 321431.770306, 44.012074)

print "Source point", a
print "Target point", b
print 'Converted: ', pyproj.transform(p1, p2, *a) # different :()

nickponline

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Re: What units are exported point clouds in for NAD27?
« Reply #6 on: September 23, 2015, 08:55:51 PM »
I guess what I'm trying to find out is:

WGS84 + h is NAD27 CA 2 + ??

Do you know the Proj file for the WGS84 transformation too?
« Last Edit: September 23, 2015, 08:57:30 PM by nickponline »

nickponline

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Re: What units are exported point clouds in for NAD27?
« Reply #7 on: September 23, 2015, 09:18:17 PM »
Looks like the two coordinate systems are:

Geographic Coordinate System: WGS 84 (EPSG::4326)
Geodetic Datum: World Geodetic System 1984 (EPSG::6326)
Ellipsoid: WGS 84 (EPSG::7030)
Prime Meridian: Greenwich (EPSG::8901)
Angular Units: degree (EPSG::9102)


Projected Coordinate System: NAD27 / California zone II (EPSG::26742)
Projection Method: Lambert Conic Conformal (2SP)
    Latitude of false origin: 37.6667
    Longitude of false origin: -122
    Latitude of first standard parallel: 39.8333
    Latitude of second standard parallel: 38.3333
    Easting of false origin: 2e+06
    Northing of false origin: 0
Geographic Coordinate System: NAD27 (EPSG::4267)
Geodetic Datum: North American Datum 1927 (EPSG::6267)
Ellipsoid: Clarke 1866 (EPSG::7008)
Prime Meridian: Greenwich (EPSG::8901)
Linear Units: US survey foot (EPSG::9003)

Does this explain the elevation difference?

Paulo

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Re: What units are exported point clouds in for NAD27?
« Reply #8 on: September 24, 2015, 01:46:00 AM »
The problem if your input heights are elisoidal, they don't really mean anything.... I would tranform them to othometric or mean sea level heights using latest geoidal model for Conus. And these I would keep fixd as Height over mean sea level remains fixed whatever datum you use....
Best Regards,
Paul Pelletier,
Surveyor

Paulo

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Re: What units are exported point clouds in for NAD27?
« Reply #9 on: September 24, 2015, 05:16:15 PM »
Here is the prj file for Nad27 / Californ ia Zone II

PROJCS["NAD27 / California zone II",GEOGCS["NAD27",DATUM["North American Datum 1927",SPHEROID["Clarke 1866",6378206.4,294.978698213898,AUTHORITY["EPSG","7008"]],TOWGS84[-10,158,187,0,0,0,0],AUTHORITY["EPSG","6267"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9102"]],AUTHORITY["EPSG","4267"]],PROJECTION["Lambert_Conformal_Conic_2SP",AUTHORITY["EPSG","9802"]],PARAMETER["latitude_of_origin",37.6666666666667],PARAMETER["central_meridian",-122],PARAMETER["standard_parallel_1",39.8333333333333],PARAMETER["standard_parallel_2",38.3333333333333],PARAMETER["false_easting",2000000],PARAMETER["false_northing",0],UNIT["US survey foot",0.3048006096012192,AUTHORITY["EPSG","9003"]],AUTHORITY["EPSG","26742"]]

The change in Z values is due mainly to shift from Clarke1866 to WGS84 elipsoid of -10 m in X, 158 meters in Y and 187 meters in Z (see TOWGS84 record....
Best Regards,
Paul Pelletier,
Surveyor