Trimble Business Center

Hello all,

I am looking for some insight into evaluating a GPS calibration report. Only to understand it better, I am not currently having any issues. The Image below shows a calibration I recently solved. I understand the scale factor, its the slope north and slope east that intrigues me. How can you tell if the slope is acceptable, fantastic, or poor? 

PPM is parts per million, which makes it simple to read for metric units, since 1 km has 1 million mm. Not sure if there is a similar relation between inch/foot/mile.

You're looking at 18 mm per km, which is 0.7 inch per 0.621 miles, which is 1.12 inch per mile.

It all depends how the geoid in your area looks like and if those control marks have elevations based on the geoid or some other average/plane coordinate system.

If they follow the geoid than your resulting slope should in theory match the geoid slope. If they're based on some kind of plane, then they should in theory be near zero or match that plane.

What you need to avoid is measuring points in a long-stretched manor, i.e. along a road. Small vertical residuals can lead to extreme plane tilts, and you may have to choose vertical shift only, instead of tilted plane.

In your case, and just looking at the numbers, I'd say you're reasonably ok for GPS use within the area of those control points. And probably double check on a few or even other ones.

How long did you observe each of those points?

Hello all,

I am looking for some insight into evaluating a GPS calibration report. Only to understand it better, I am not currently having any issues. The Image below shows a calibration I recently solved. I understand the scale factor, its the slope north and slope east that intrigues me. How can you tell if the slope is acceptable, fantastic, or poor? 

Ronny,

I sat on each point for 30 seconds.

PPM is parts per million, which makes it simple to read for metric units, since 1 km has 1 million mm. Not sure if there is a similar relation between inch/foot/mile.

You're looking at 18 mm per km, which is 0.7 inch per 0.621 miles, which is 1.12 inch per mile.

It all depends how the geoid in your area looks like and if those control marks have elevations based on the geoid or some other average/plane coordinate system.

If they follow the geoid than your resulting slope should in theory match the geoid slope. If they're based on some kind of plane, then they should in theory be near zero or match that plane.

What you need to avoid is measuring points in a long-stretched manor, i.e. along a road. Small vertical residuals can lead to extreme plane tilts, and you may have to choose vertical shift only, instead of tilted plane.

In your case, and just looking at the numbers, I'd say you're reasonably ok for GPS use within the area of those control points. And probably double check on a few or even other ones.

How long did you observe each of those points?

Hello all,

I am looking for some insight into evaluating a GPS calibration report. Only to understand it better, I am not currently having any issues. The Image below shows a calibration I recently solved. I understand the scale factor, its the slope north and slope east that intrigues me. How can you tell if the slope is acceptable, fantastic, or poor? 

I've only performed one site calibration, so I'm assuming this report is from TBC?  Did you observe those control points only once?  Since I'm assuming you're using TBC, I would have observed each of those points twice using the same point name each time choosing "store another" when prompted and choosing to -"reset satellites" in GNSS initialization in Trimble Access.  And if the second observation was off by more than you think it should have been - then observe it a third time. That would improve your results since those points would have been averaged as you brought your JOB file into TBC.

Ronny,

I sat on each point for 30 seconds.

PPM is parts per million, which makes it simple to read for metric units, since 1 km has 1 million mm. Not sure if there is a similar relation between inch/foot/mile.

You're looking at 18 mm per km, which is 0.7 inch per 0.621 miles, which is 1.12 inch per mile.

It all depends how the geoid in your area looks like and if those control marks have elevations based on the geoid or some other average/plane coordinate system.

If they follow the geoid than your resulting slope should in theory match the geoid slope. If they're based on some kind of plane, then they should in theory be near zero or match that plane.

What you need to avoid is measuring points in a long-stretched manor, i.e. along a road. Small vertical residuals can lead to extreme plane tilts, and you may have to choose vertical shift only, instead of tilted plane.

In your case, and just looking at the numbers, I'd say you're reasonably ok for GPS use within the area of those control points. And probably double check on a few or even other ones.

How long did you observe each of those points?

Hello all,

I am looking for some insight into evaluating a GPS calibration report. Only to understand it better, I am not currently having any issues. The Image below shows a calibration I recently solved. I understand the scale factor, its the slope north and slope east that intrigues me. How can you tell if the slope is acceptable, fantastic, or poor? 

Hello all,

I am looking for some insight into evaluating a GPS calibration report. Only to understand it better, I am not currently having any issues. The Image below shows a calibration I recently solved. I understand the scale factor, its the slope north and slope east that intrigues me. How can you tell if the slope is acceptable, fantastic, or poor?