Typically I would understand a geodetic distance to be an ellipsoidal distance which is different to a grid or ground distance.
Florida is low elevation, and you may have been in a location where the SPC scale factor is close to 1.00000
The poster is in Wisconsin at a higher elevation and we don't know where relative to his SPC.?ÿ So the two experiences are likely to differ.
@lukenz?ÿ
heres a great summary of the lot of them:
https://amerisurv.com/2013/08/23/ground-versus-grid-low-distortion-projections-part-1/
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god Im going to miss working in Just one LDP when I start my new job next week......
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??? ??? ???®?ÿ
@lukenz?ÿ
I'm curious about the wisdom of Topcon for including the geodetic distance in their point-to-point inverse output. How is that data useful to me?
Because we are working in plane trig. Aka flat earth geometry. And, when you integrate this with lat Lon, and geoid separation, then:
There are places where grid dist and ground dist are equal.
There are places where grid dist is shorter than ground.
There are places where grid is greater than ground.?ÿ
Imagine a basketball. With plane of glass, cutting a little off the ball.
Glass is grid. Flat earth.
Grid to surface where it cuts off a little of the ball, means grid to ground is more than one. Ie 1.0010 multiplication factor.
Where the pane of glass intersects the surface, it's 1.0000 Or equal.
Where the glass is above the surface, ground dist is less than grid.
It's stuff that graphics help with. (Draw me a worksheet)
In hill country, you can have both.
Now what?
N
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Developable Surfaces
Inorder to "really" understand "grid" Projections (Lambert, Transverse Mercator, etc), one has to get their head around the basic geometry involved.
https://en.wikipedia.org/wiki/Developable_surface
From what I gather on here you guys in the USA are only interested for grid/ground distances for boundary/topo work. Here in New Zealand (and in Australia I believe) we show distances as ellipsoidal on our survey plans;
Area = ellipsoidal
Bearings = grid
Coordinates = grid
Distances = ellipsoidal
This approach has advantage of grid bearings making all nearby surveys consistently orientated to official projections and grid coordinates make calculations easier than lats/longs. Having ellipsoidal distances gives pretty consistent distances/areas everywhere in country (elevated areas are scaled down to ellipsoid/"sea level").
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Long winded way of saying that geodetic/ellipsoidal distances for cogo may not be needed in your locality if your not doing geodetic control work but in other parts of the world it's a very useful/necessary feature.
Here is a quick Florida calculation:
Two points on the Florida East Zone
On the Central Meridian for each point of long=W81d, at lat N27d and N27d02'
both are at sea level to make the picture easier to understand.
inverse between them:
Grid and Geo azimuth 0d00'00"
Grid distance=12,116.912
Ellipsoid distance=12,117.625
Ground distance=12,117.576
elevation for both points=0.00
ellipsoid height for the north point=-83.85?ÿ south point=-83.38
numbers are USFT.?ÿ
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You can tell from the numbers where the ellipsoid is. Is it above or below you?
The radius of the earth is about 20,000,000' which means 1ppm corresponds with 20' of elevation change. As you can see there is 80' in the geoid height which should create 4ppm for the elevation scale. 12,117.576x.000004=0.048',,,,,12,117.576+.048=12,117.624 or very close to the ellipsoid length using rough calcs.?ÿ
Now you know if the ellipsoid is above you or below since the ellipsoid length is longer.?ÿ
And you can back in all your scale factors.?ÿ
@lukenz?ÿ My procedure is thus: On the rover rod, always the same rod height of 5.6 feet, with a bipod, always facing the same direction on each control shot (usually north, but it could be a distant radio tower), take the five shots in about one minute total, which means the Leica has automatically re-initialized between each shot. That procedure pretty much means the little bit of bubble error that might be there is the same for each point, so everything is relatively accurate. With the reflectorless there is no rod error because I shoot a target on the ground 0.10 feet above the mag nail (I use a painted and scribed concrete brick for the sight). Ta da!
@bruce-small Or it's a really long line.?ÿ In East TN typical grid to ground difference is .01 per 100' so a 1000' line on grid would tend to produce an answer 0.10' different it shot with GPS on Grid and a TS on ground coordinates.
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?ÿHere is a residual report from StarNet. These are points tied by GPS-RTK using a Topcon GR-3 and subsequently traversed. Typical of what I do. There are about 50 points traversed and side tied in this network and these dozen tied by GPS.?ÿ ?ÿ ?ÿ?ÿ
You've caused me to go to Javad's website, where they are good enough to clearly list their prices. Which is refreshing, I must say.?ÿ But I am holding a quote on a pair of BRx7s, with data collector, that is right there in the same price range. Just sayin'.?ÿ?ÿ