rlshound, post: 322896, member: 6800 wrote: Hello,
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
Paul. It's been almost 10 years, but I did this on a Maricopa county survey that reported everything in NAD83 (96??? I forget the realization) State Plane coordinates. I calibrated to one point, and checked into other handful of reported positions on the survey within a couple hundredths. I felt pretty darn confident.
In terms of the geodetic north issue, you shouldn't see a difference in terms of rotation, since that is really dependent on where you are in the zone geographically when using the same datum they used. Realistically, your "true" position probably isn't much more than few feet different at most, and the rotation and convergence is negligible (0.00000000000000000000000001 micro-radians likely) over that short of a distance.
rlshound, post: 322896, member: 6800 wrote: Hello,
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 completely agree with Kris. I too used to do the single calibration in Trimble and no longer do. If the county has that extensive of a system, then they probably have a projection based on that system or a LDP can be created based on it. Then all you need to do is use that projection in the collector and set up on the point - no callibration/localization (Trimble/Leica). This way you have a consistent projection for all of the work done in the county. You can then inventory all of your work in one single geodatabase making research and cross-referencing of jobs very efficient.
If you are tying into an already controlled system, then leave the office with that projection set in your DC. Either set on a momument or at the worst set a control point and tie one then shift over to the system (checking into others), but I would never calibrate or do a one point control. That just causes problems and you also lose the ability to use the system correctly. It may be that the system isn't all that great, but what I've almost always found that if it's DOT or a different agency, getting on the control system is the way to go. Imagine that you have a county system of control, then you calibrate to one or two nearby points, how can that ever be a good idea?
SurveyAK, post: 324226, member: 9968 wrote: Paul. It's been almost 10 years, but I did this on a Maricopa county survey that reported everything in NAD83 (96??? I forget the realization) State Plane coordinates. I calibrated to one point, and checked into other handful of reported positions on the survey within a couple hundredths. I felt pretty darn confident.
In terms of the geodetic north issue, you shouldn't see a difference in terms of rotation, since that is really dependent on where you are in the zone geographically when using the same datum they used. Realistically, your "true" position probably isn't much more than few feet different at most, and the rotation and convergence is negligible (0.00000000000000000000000001 micro-radians likely) over that short of a distance.
Thanks SurveyAK....the consensus is agreeing with you that you will not see the distortion withing such a small area....spent some time in Anchorage, great place....worked on the Westchester lagoon project...liked the people...
See You, Paul
MightyMoe, post: 324254, member: 700 wrote: If you are tying into an already controlled system, then leave the office with that projection set in your DC. Either set on a momument or at the worst set a control point and tie oneH then shift over to the system (checking into others), but I would never calibrate or do a one point control. That just causes problems and you also lose the ability to use the system correctly. It may be that the system isn't all that great, but what I've almost always found that if it's DOT or a different agency, getting on the control system is the way to go. Imagine that you have a county system of control, then you calibrate to one or two nearby points, how can that ever be a good idea?
Thanks MightyMoe...I appreciate your advice
geonerd, post: 324228, member: 8268 wrote: I completely agree with Kris. I too used to do the single calibration in Trimble and no longer do. If the county has that extensive of a system, then they probably have a projection based on that system or a LDP can be created based on it. Then all you need to do is use that projection in the collector and set up on the point - no callibration/localization (Trimble/Leica). This way you have a consistent projection for all of the work done in the county. You can then inventory all of your work in one single geodatabase making research and cross-referencing of jobs very efficient.
O
Hello geonerd, thanks for the ideas..........do you know any references for creating ldps?
I am almost 100% certain that there are extensive LDP in AZ created under the direction of Michael Dennis who is Mr. LDP, he assisted with creating the zones for Oregon as a consultant to ODOT, I was on the Technical Development Team. There is some good info in the OCRS Handbook and User Guide that should be good background information for anyone, not just those using the OCRS.
If you are working in an area that has a usable zone of any kind that you are trying to align to, be it a LDP, SPC, UTM, whatever, then there should NEVER be a need to create some one off localized system, that is bad practice. The one point idea was for working on a local system over a very small geographic area, it allows you to jump out, find a monument, call it 5000/5000, get a GNSS position, then start finding other local mons, it was created as nothing more than a way for folks buying GPS equipment in the earlier days to go to work without knowing anything about geodesy or formal coordinate systems. Mr. Dennis has prepared a workshop entitled "GPS, Geodesy, and the Ghost in the Machine" that would be good education for all of us to attend. Not sure if other presentations are scheduled, BUT it was presented in Ontario, CA just this week.
SHG
rlshound, post: 324532, member: 6800 wrote: Hello geonerd, thanks for the ideas..........do you know any references for creating ldps?
Shawn Billings isn't the only one, but he is my go-to about LDP. I would definitely talk to him. He likes that stuff way more than me. That's why I keep it on the grid.
Low Distortion Projections Part 1 and 2 from American Surveyor Magazine.
rlshound, post: 324532, member: 6800 wrote: Hello geonerd, thanks for the ideas..........do you know any references for creating ldps?
Michael Dennis is the man - and I believe he lives down there by you somewhere. Check the Board or with NGS for contact data.
Right your are about Michael Dennis. His company is Geodetic Analysis, LLC. You can find the website by Google search on the company name. He's currently on sabbatical from NGS, working on his doctorate at Oregon State.
Shawn's articles are great and mine shows an application. In addition, Dr. Charles Ghilani has a couple of articles this year in XYHT.
Here's a great introductory reference by Michael Dennis: http://www.iowadot.gov/rtn/pdfs/IaRCS_Handbook.pdf
Maricopa County is one of the first that Mr. Dennis did, so it's 100% certain that an LDP exists there.
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.
Kris,
I agree that the distortions associated with a single point calibration can get out of hand very quickly. That being said, every conformal projection by definition has convergence except upon its central meridian and the resulting grid azimuth is equal to geodetic azimuth only said central meridian.
John Putnam, post: 324879, member: 1188 wrote: Kris,
I agree that the distortions associated with a single point calibration can get out of hand very quickly. That being said, every conformal projection by definition has convergence except upon its central meridian and the resulting grid azimuth is equal to geodetic azimuth only said central meridian.
I totally agree with that. The issue is, when you do the 1 point calibration, you don't know what the parameters are, so you don't know how to adjust data. You just think that North is North, and it isn't. That's why these are so dangerous. Small tracts, up to say, 100 acres, are negligible. Over that, and it gets to be a hat trick.
The reason I love the grid is that I NEVER know just how far my project will go. So, building a LDP for say an oil & gas project, is difficult at best because I never know just how much land is leased, or is planned to be put into production. In existing fields, you never see it all. So the grid doesn't fail me and I'm always working in the same system. If one were to try this with the one point thingy, in one of my fields, and the smallest is 5 miles by 3 miles, then you're going to have problems tying together and keeping them right. If one were to know ahead of time, I agree the LDP is great. But, my work is typically done at less than 800 feet MSL and I don't care that my bearings are some 2° different than true. It's easy enough to calculate.
rlshound, post: 324532, member: 6800 wrote: Hello geonerd, thanks for the ideas..........do you know any references for creating ldps?
I've had these two bookmarked for a while:
What is an LDP?
and
Custom Ellipsoids & LDPs
Kris Morgan, post: 325029, member: 29 wrote: I totally agree with that. The issue is, when you do the 1 point calibration, you don't know what the parameters are, so you don't know how to adjust data. You just think that North is North, and it isn't. That's why these are so dangerous. Small tracts, up to say, 100 acres, are negligible. Over that, and it gets to be a hat trick.
The reason I love the grid is that I NEVER know just how far my project will go. So, building a LDP for say an oil & gas project, is difficult at best because I never know just how much land is leased, or is planned to be put into production. In existing fields, you never see it all. So the grid doesn't fail me and I'm always working in the same system. If one were to try this with the one point thingy, in one of my fields, and the smallest is 5 miles by 3 miles, then you're going to have problems tying together and keeping them right. If one were to know ahead of time, I agree the LDP is great. But, my work is typically done at less than 800 feet MSL and I don't care that my bearings are some 2° different than true. It's easy enough to calculate.
I haven't much use for calibrations. I do not agree that you 'do not know your distortions. Every brand I've used stores an easily accessed set of transformation parameters derived during the localization. The math behind them is elementary. Anyone remotely familiar with local conditions can spot problems with no great effort.
thebionicman, post: 325100, member: 8136 wrote: I haven't much use for calibrations. I do not agree that you 'do not know your distortions. Every brand I've used stores an easily accessed set of transformation parameters derived during the localization. The math behind them is elementary. Anyone remotely familiar with local conditions can spot problems with no great effort.
Pure poppycock! At least with our 5700's. When the "No projection/No Datum" is selected and a here position made for the base, and then calibrated to 10,000/10,000, no where in my DC does it show semi major/minor axis, central meridians, et cetera. Therefore, unless you take the "autonomous" LLH at selected points and run them through some software (I have many that I could), then you can't begin to understand how much "theta" your projection is generating and at what distances or how scale is affecting the distances. It's just not modeled in the DC, which is what, from my observation, most people who use this method, are working with.
I'm not talking about Helmert 7 point transformations or a host of other "modeled" calibrations to existing control where it has to yield some residuals for the user to continue, I'm talking about the "geodetic bearings and surface distances" that we were "taught" from the representatives. They didn't explain the pitfalls. It was only after I understood the "elementary math" and began to examine "local conditions" that I understood just how bad these can get, especially on large tracts traversing with base stations.
I've used Trimble, Leica and Topcon. The localization parameters for all 3 were easy to find and evaluate. While it is possible they have hidden these files better, I doubt they would do so. Spend some time looking at the files created by your dc and crashing around the obscure menus. If the information isn't there I would put it on eBay.
Another interesting discussion....thanks for the references to low distortion networks...I'll look into them....been working alot....Thank you all for taking the time to respond....Paul
Alan Chyko, post: 325097, member: 1145 wrote: I've had these two bookmarked for a while:
What is an LDP?
and
Custom Ellipsoids & LDPs
Hi Alan,
Just found your question. I do have a powerpoint I had created for working with projections between CAD, GIS, and surveying. It's not a "final" product since it has not been taught yet, but it does include a section on LDPs. More than happy to send you a copy - contact me if you want a copy and let me know how to send it to you. Paul can say whether it's worth looking at;-)
and I agree with Loyal's statement that customizing an ellipsoid just adds complexity - I prefer the KISS philosophy. When creating an LDP, project from a standard ellipsoid.
