1 point calibration ?

What software? Since you said 'calibrate' I'm guessing Trimble. Most software allows you a way to use either geodetic North or grid north per whatever coordinate system you are surveying in. I'm not familiar enough with Trimble software to say for sure.

I use them from time to time.
Basically, it gives you a data set that is tied together, but not to anything else.

I will always hit a couple of monuments and benchmarks, so I can translate the coordinates if need be.

A single point calibration will put you on geodetic north and does a TM projection with the origin at the calibration point. Depending on your elevation things can go south pretty rapidly, I definitely wouldn't go more than about a mile with it. There are better ways to get into ground coordinates.

Its been a while since I have used Trimble but more fluent with Leica GNSS. I use 1 Point Localization often; perfect for tying into a local NGS monument. If you set up your job with state plane projections, then its a simple shift (no rotation required).

It's a terrible idea. What most don't get is that when the "one point calibration" is done, you effectively create a mapping projection complete with semi-major/minor axis, angles of convergence and scale factors. Relevant range become VERY critical and at some point, North isn't geodetic North and GPS distances aren't surface but grid. The issue is that EACH one is different and requires that you use the same base point to maintain is integrity or don't go very far since everything goes to hell quick (about a mile).

We used to do these and now we don't. Live and learn but I can't believe 20 years after GPS was readily available that ANYONE would continue to use this method because of the pitfalls inherent with the procedure.

It should be noted that one point calibrations are NOT Low Distortion Projections and that if you want true bearings and surface distances from the black box, one should really understand what is going on and build a LDP for their area. Otherwise, put it on the grid and then you have all of the flexibility to do whatever you want and need with the data in an unmolested format.

I prefer to collect the "raw" data in the field and then do any translating/rotating/massaging back in the office later.

Brad Ott, post: 322929, member: 197 wrote: I prefer to collect the "raw" data in the field and then do any translating/rotating/massaging back in the office later.

Roger that!

The only time we do a 1 point calibration is after we've already set up using an autonomous position, we find a point with good coordinates. In this case, is the calibration not just a simple translate?

Tommy Young, post: 322953, member: 703 wrote: The only time we do a 1 point calibration is after we've already set up using an autonomous position, we find a point with good coordinates. In this case, is the calibration not just a simple translate?

Only inasmuch as if you have an autonomous position on the grid that is CLOSE to the real value as posted by OPUS. Then, pragmatically, yes it is a translation. However, in theory, ALL autonomous positions require SOME form of rotation due to angles of convergence, even on the grid. Most of the time, the autonomous position is less than 10 feet away so the rotation is moot being on the scale of less than a quarter of a second in my area so I translate only.

If we still had to contend with spoofing, then it may very well require a rotation THEN translation.

Here in Maricopa County, Arizona the county surveyors did an extensive inventory of all PLSS monuments, GDACS. Having this information is very helpful and it is easily accessed via the county's website. It is also reliable...guess I should have mentioned that first. Given the amount of reliable information to check into I would like your input on 1 point calibrations. Do you use them? Why or why not? Also with a 1 point I believe it sets you at geodetic north which requires rotation back + or - depending on where you are in the zone? Any non political advice would be appreciated.

Thanks, Paul

I guess I'm a little confused. I have to assume that this extensive inventory of PLSS monuments is on some kind of standard projection, definitely not a local grid, so why you even consider doing a single point calibration?

I do single point calibrations/localizations occasionally, but always save the raw files to be post processed back in the office into a standard projection. By keeping them in a local grid system I'm severely limiting their potential value. I agree with Kris that without really understanding the limitations of a single point localization, things can get hosed up in a hurry.

Shawn Billings, post: 322907, member: 6521 wrote: What software? Since you said 'calibrate' I'm guessing Trimble. Most software allows you a way to use either geodetic North or grid north per whatever coordinate system you are surveying in. I'm not familiar enough with Trimble software to say for sure.

Hello Shawn, yes Trimble software, Thanks

RFB, post: 322908, member: 142 wrote: I use them from time to time.
Basically, it gives you a data set that is tied together, but not to anything else.

I will always hit a couple of monuments and benchmarks, so I can translate the coordinates if need be.

Thats the reason I mentioned the abundance of reliable monuments....Thats what I've done in the past, Thanks

Every tool has its place. You absolutely can use a predetermined projection and geoid with a 1 point localization. The difference is there will be a translation applied on top of it. If you want to discuss it email me at the address in my profile

Lee D, post: 322917, member: 7971 wrote: A single point calibration will put you on geodetic north and does a TM projection with the origin at the calibration point. Depending on your elevation things can go south pretty rapidly, I definitely wouldn't go more than about a mile with it. There are better ways to get into ground coordinates.

Thanks Lee, the times we have used the one point is within a section....using an averaged scale factor to get it to ground, would be interested in the other means you've mentioned, Thanks Again, Paul

benchmark11, post: 322921, member: 2078 wrote: Its been a while since I have used Trimble but more fluent with Leica GNSS. I use 1 Point Localization often; perfect for tying into a local NGS monument. If you set up your job with state plane projections, then its a simple shift (no rotation required).

Thanks Shawn, ok...if you've set to stateplane, no rotation...ok...Thank You, Paul

Kris Morgan, post: 322924, member: 29 wrote: It's a terrible idea. What most don't get is that when the "one point calibration" is done, you effectively create a mapping projection complete with semi-major/minor axis, angles of convergence and scale factors. Relevant range become VERY critical and at some point, North isn't geodetic North and GPS distances aren't surface but grid. The issue is that EACH one is different and requires that you use the same base point to maintain is integrity or don't go very far since everything goes to hell quick (about a mile).

We used to do these and now we don't. Live and learn but I can't believe 20 years after GPS was readily available that ANYONE would continue to use this method because of the pitfalls inherent with the procedure.

It should be noted that one point calibrations are NOT Low Distortion Projections and that if you want true bearings and surface distances from the black box, one should really understand what is going on and build a LDP for their area. Otherwise, put it on the grid and then you have all of the flexibility to do whatever you want and need with the data in an unmolested format.

"one should really understand what is going on and build a LDP for their area. Otherwise, put it on the grid and then you have all of the flexibility to do whatever you want and need with the data in an unmolested format." Thanks Kris for your time and insight, will work on LDP generation...Paul

rlshound, post: 323139, member: 6800 wrote: Thanks Lee, the times we have used the one point is within a section....using an averaged scale factor to get it to ground, would be interested in the other means you've mentioned, Thanks Again, Paul

When I first read your question, I was thinking in terms of a single point calibration into an assumed system, i.e., 10000,10000. In that case you get a local TM aligned to geodetic north. As Kris pointed out, there would be convergence, but in a small area it would be negligent. We used to use that method regularly prior to the availability of OPUS and CORS that we have now. Using that method in Trimble Access you would start the job with No Datum / No Projection and then go to site calibration.

However, you can also start a project in Access in SPCS and then do a single point calibration to a known monument. In that case it is just a simple translation; your base coordinates are going to be close enough that there won't be any distortions introduced. People do that here when using the VRS network - if you're using VRS but need to hold a monument you can do a single point calibration; you're typically only translating out a tenth or so in that case.

The other means I was referring to was to start a job in SPCS and change the coordinates from Grid to Ground; you can then use a point in the project to define the combined scale factor. Again, I wouldn't do that over a large area, especially if there were large elevation changes. A lot of people will caution against the potential pitfalls of doing that; some of these guys who survey out west have geodetic issues that we simply do not have to worry about when surveying in a state that's at sea level and doesn't have much elevation change.

Maricopa County has a much-written-about county-wide low distortion projection. Is it not workable?

Williwaw, post: 322961, member: 7066 wrote: Here in Maricopa County, Arizona the county surveyors did an extensive inventory of all PLSS monuments, GDACS. Having this information is very helpful and it is easily accessed via the county's website. It is also reliable...guess I should have mentioned that first. Given the amount of reliable information to check into I would like your input on 1 point calibrations. Do you use them? Why or why not? Also with a 1 point I believe it sets you at geodetic north which requires rotation back + or - depending on where you are in the zone? Any non political advice would be appreciated.

Thanks, Paul

I guess I'm a little confused. I have to assume that this extensive inventory of PLSS monuments is on some kind of standard projection, definitely not a local grid, so why you even consider doing a single point calibration?

I do single point calibrations/localizations occasionally, but always save the raw files to be post processed back in the office into a standard projection. By keeping them in a local grid system I'm severely limiting their potential value. I agree with Kris that without really understanding the limitations of a single point localization, things can get hosed up in a hurry.

Hello Williwaw, thanks for your input "I agree with Kris that without really understanding the limitations of a single point localization, things can get hosed up in a hurry" this is the reason for the question....thats why I value this site and the willingness of its members to share their knowledge and experience...Thanks, Paul