Is idea of scaling a NAD27 grid distance to ground using a 27 combined scale factor then scaling it to NAD83 Grid using an 83 combined scale factor sound? Seems to give different results than transforming the coordinates but I think it is what I am seeing someone having done here.
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tomchurch, post: 443159, member: 10174 wrote: Is idea of scaling a NAD27 grid distance to ground using a 27 combined scale factor then scaling it to NAD83 Grid using an 83 combined scale factor sound? Seems to give different results than transforming the coordinates but I think it is what I am seeing someone having done here.
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The only difference (as long as they didn't change the coordinate system parameters) is that NAD27 distances were reduced to "sea level" while NAD83 distances are reduced to the ellipsoid. This was done because they didn't really know where the ellipsoid was located until the 60's or 70's, and then only roughly. Difference is about 1 ppm per 6 m of ellipsoid separation.
There can be yuge differences between a state plane distance in 27 and 83. You need to know the parameters
All I have is a map with coordinates to two decimal and distances to two places saying it is in 27. I computed a 27 scale factor using Carlson to scale to grid.
tomchurch, post: 443167, member: 10174 wrote: All I have is a map with coordinates to two decimal and distances to two places saying it is in 27. I computed a 27 scale factor using Carlson to scale to grid.
It should work, but the resulting state plane distance can be quite different. It depends on how each zone was set-up.
[USER=700]@MightyMoe[/USER] That would be a good thing...if I use the translation of the line I miss two called points (located in 83) by just under 5 feet. If I scale it using this method I hit them within 0.25 over miles.
Unless that line is really long I don't see how .25 miles is appearing.
It's a 15ish mile line east west.
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[USER=700]@MightyMoe[/USER] Woops...to clarify it is hitting the monuments located in 83 within 0.25' over 15 miles when I use the scale factor method...vs. 5'ish using the transformation.
Which 83 are you using? Was translating 27 control points to our opus solution compared to a city gis grid thats supposedly 83 but no one knows what version it's on and was getting around that fluctuation trying from 83(86) up thru the 83(2011). That was using the conversion tool on the NGS site.
MightyMoe, post: 443163, member: 700 wrote: There can be yuge differences between a state plane distance in 27 and 83. You need to know the parameters
A pair of NAD27 points, inverse = grid. Apply '27 combo factor = ground.
Ground, apply '83 combo factor = '83 grid.
Not seeing an issue.
However, if the pair of pts are very far apart, then special consideration to determine the correct scale factor.
Please check me: grid factor at pt-1, plus grid at factor at pt-2, plus 4x the grid factor at middle, divide by 6.
It's been a long time.
Found it ....
Page 9, near the bottom. Center weighted grid factor, for proper grid factor of long lines. Might make up for your difference.
http://www.ejsurveying.com/uploads/2/5/6/6/25668328/working_with_grid_coord.pdf
John Hamilton, post: 443161, member: 640 wrote: ... NAD27 distances were reduced to "sea level" while NAD83 distances are reduced to the ellipsoid.
In other words there is no distinction made between ellipsoid heights and orthometric heights in NAD 27. In practice the field procedures used to determine NAD27 coordinates usually contained enough error that you can safely ignore effects of geoid seperation on grid/ground scaling. But there are some cases where modern methods have been used to determine NAD27 positons, then those things come into play.
In a majority of cases the '27 zone parameters are the same as the '83 zone parameters, except for false northings/eastings, in which case grid distances (not coordinates) in both are going to be the same.
Mark Mayer, post: 443236, member: 424 wrote: In other words there is no distinction made between ellipsoid heights and orthometric heights in NAD 27. In practice the field procedures used to determine NAD27 coordinates usually contained enough error that you can safely ignore effects of geoid seperation on grid/ground scaling. But there are some cases where modern methods have been used to determine NAD27 positons, then those things come into play.
In a majority of cases the '27 zone parameters are the same as the '83 zone parameters, except for false northings/eastings, in which case grid distances (not coordinates) in both are going to be the same.
NAD 27 is a best fit to the continent, making geoid ?? NGVD29
I think the center weighting grid factor may be significant.
NAD27 used the clarke 1866 ellipsoid, and made H=h (i.e. N=0) at the origin (MEADES RANCH). The problem was they had no way to accurately compute ellipsoidal heights or geoid separations until satellites came along in the late 50's/early 60's. And of course it is necessary to reduce distances to the ellipsoid for geodetic computations. There were major scale biases present in the western US because of this "omission". Here is a geoid map for NAD27 produced in 1974 (1 m contours):
John Hamilton, post: 443248, member: 640 wrote: NAD27 used the clarke 1866 ellipsoid, and made H=h (i.e. N=0) at the origin (MEADES RANCH). The problem was they had no way to accurately compute ellipsoidal heights or geoid separations until satellites came along in the late 50's/early 60's. And of course it is necessary to reduce distances to the ellipsoid for geodetic computations. There were major scale biases present in the western US because of this "omission". Here is a geoid map for NAD27 produced in 1974 (1 m contours):
And didn't the USC&GS originally hold 0.000 MSL for both coasts? causing a significant tilt.
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Do you recall center weighted scale factor for long lines?
They did force NGVD29 0.00 to be equal to "sea level" at multiple tide stations on both coasts, but that was a leveling issue.
For reduction of distances in the horizontal (triangulation) network, I think they just assumed N=0, which meant that on the west coast (30 m separation) there was a 5 ppm bias in distances.
I would be very careful about applying a "one size fits all" method to any NAD27/NAD83 manipulation. There is more to it than meets the eye (as some have pointed out above).
I would download a couple of NGS Points around your area, that have BOTH NAD27 & NAD83 values published on them, and run some computations in BOTH Datums. Depending on where you are, how big your project is, and which values you are planning on switching between, you may find that BOTH the scale and rotation may be larger than you might think.
Loyal
Take a couple of geographic points anywhere in central Montana, put an elevation of 3000' on them, then reduce both to 27 and 83; 83 will be shorter by quite a bit. Other zones I work in don't have that issue, but I would'nt assume anything,,,,,
