Grid and Ground Distances

[Okiryu]

Hi, what is the difference between grid and ground distances? If I have coordinates between two points (measured by a GPS) and measured the lenght between these points, is that considered ground distance?

 

[racookpe1978]

How far apart are the two distances?

How accurate do you need your distance?

 

[Okiryu]

Around 60 meters, but this distance was measured with a total station. The GPS coordinates were affected with a correction factor, so I am assuming that these distances are ground distances. Is this correct ?

 

[racookpe1978]

No elevation change?

 

[Okiryu]

Elevations changed a bit, since GPS Elevations were also affected by the correction factor.

 

[racookpe1978]

See EM 1110-1-1005
1 Jan 07
5-10. Grid Elevations, Scale Factors, and Convergence
In all planer grid systems, the grid projection only approximates the ellipsoid (or roughly the ground), and “ground-grid” corrections must be made for measured distances or angles (directions). Measured ground distances must be corrected for (1) elevation (sea level factor), and (2) ground to grid plane (scale factor).
Figure 5-9 below illustrates a reduction of a measured distance (D) down to the ellipsoid distance (S). Not shown is the subsequent reduction from the ellipsoid length to a grid system length. Observed directions (or angles) must also be corrected for grid convergence. Also shown on the figure is the relationship between ellipsoid heights (h), geoid heights (N), and orthometric heights (H).

If there is no change in elevation, over 60 meters, just how much change are you expecting based on the earth's nominal radius of 7000 odd kilometers?

 

[Okiryu]

racookpe1978, thanks for taking the time on this. Our coordinates and elevations for benchmarks determined by GPS were corrected using correction factors to account for the sea level and scale factors. After corrected, we set up other benchmarks using a total station. I think that since we have corrected our original BMs (determined by GPS) and then established new BMs using a total station, our distances are ground distances. Thanks again for your replies.