All true if you're doing it right to begin with. Shouldn't take any detective work at all!
But it does. Used to be you needed a control network of some sort to get on state plane, NGS mons for example, or later maybe your own static GPS network. Takes some book learning, brains, and effort to get on state plane and just the fact that it's a network figure means your state plane coordinates are probably good relative to each other and maybe a small part of the greater world.
The RTK proliferation has made it possible to plop a state plane coordinate on anything you want. Never mind if the data collector is set to grid or ground or pseudo-state-plane-project-ground. No network figure, no redundancy over time. Hold two for bearing with your total station, shoot the distance on the ground and note it as "off" a little bit. Continue the job "on state plane." Those coordinates are going straight into CAD. No check ins on known coordinates / known datum. Sure it's bad technique, bad surveying, no redundancy, etc. But it is what people do. An equipment vendor once told me that surveyors only buy about 10% of the survey grade equipment...
within that 10%, technology has outpaced the learning curve. It has made it possible to perform survey-like motions without wanting to understand the inner workings. It has taken an elegant and useful system like state plane and turned it into gibberish not to be trusted until it is deciphered.
Just as a practical thing, seems better to have a few ways to decipher the gibberish. We're not having much luck making everyone do it right "just because".
Another 0.04', time to make coffee.
Another point to consider when tying into State Plane Coordinates is that they are walking along over time. OPUS/CORS is more or less usless if you are trying to check into state plane that is too old.
The passive network in this area has gone through 4 adjustments now and if the superseded control wasn't published on the datasheets there wouldn't be much hope to check an older state plane job, unless like I have you kept records of each new coordinate.
I'm starting a project in a few weeks with 41 existing control points and the first job item is to check the control. OPUS/CORS will be a waste of time beyond just getting a rough idea if the points are in the ballpark of state plane (no the points can't be "updated").
I'm expecting to see about a 2 feet east-west shift because they are on NAD83 (1986).
But explain all this to someone else who thinks OPUS is gospel-it can get frustrating. Then add GEOID models and WHEW!
I'm really interested to see how this control matches the new GEOID model for slope. All 41 points were leveled and traverse almost 10 miles so I'll get a nice look at real numbers.
Kind of weird that we want to use SPC's yet express survey data in the real ground based distances and earth based bearings. Maybe one shouldn't be using SPC's for their surveys. Sometimes the way to make things simple is to just make them simple. I believe its better to just use an LDP which gets it near to a plane survey on the ground. Then provide the data that would enable someone that needs SPC or UTM or even a different LDP to go there. Actually, once understood, it is simple.
> I'm expecting to see about a 2 feet east-west shift because they are on NAD83 (1986).
So, are you thinking you won't be able to find the control points? :>
The most important elements to me are (a)that the orientation of the project coordinate grid is well known and (b) the projection scale is well known. You're probably going to need to survey three or four control points just to make sure it wasn't established with some funky RTK, maybe more if the random errors in the control flood the systematic errors too much. But you'd have to do that regardless of which projection was used.
The most important point to me is the ease with which you can plug the control point coordinates into a control, pick a standard projection from a library in the controller, and navigate right to the point. If someone has listed the coordinates on a plan or document, there is no intermediate jacking around required. You have what you need to proceed.
:good:
I'm really beginning to think you're right, LR. Presently, we use SPC's to tack the map to the globe, but the bearings and distances we report look just about like they would have when Dad opened shop in the early 80's. We use SPC's because it's kind of "en vogue", but we could just as easily use a UTM staple gun or a Lat Long thumb tack or a LDP sticky-tack to put that map on the globe and it would still stick right where it was supposed to be. Technically, SPC's are flawless and work as well as they did nearly 100 years ago. The baggage of misuse they tote around though has gotten pretty cumbersome.
The point of this thread is that retracing a survey done with some geographic reference is not a whole lot different in the office than retracing a modern survey without a geographic reference. Most folks are going to enter the bearing/distance calls from the survey into the computer or data collector. Then they'll translate it to geographic if it's available and head out into the field with the calculated coordinates. So the flavor of geographic coordinates, or whether the bearings and distances directly relate to the reported coordinate system or indirectly relate to the coordinate system (apply mapping angle and scale factor) really doesn't matter, so long as the surveyor is equipped and knowledgeable enough to transform them into his or her system of choice.
The most important point to me is the ease with which you can plug the control point coordinates into a control, pick a standard projection from a library in the controller, and navigate right to the point
I'm expecting to find everything; I'll occupy the four given static points and tie the other 37 with PPK. I'm only going to use CORS ties for a very rough check.
The existing control is fixed and can't change.
The important thing is that the 41 control points are still in the ground and that they haven't been disturbed.
I'm not reinventing the wheel for this survey. The data collector will be loaded with all the points and the projection before leaving the office.
I expect it all to fit.
I'm really more interested in the GEOID numbers than anything else. These points were all leveled and that should be interesting-levels compaired to GEOID12.
> The existing control is fixed and can't change.
Well, it will have to if a control point has been disturbed or shifted, right?
> The important thing is that the 41 control points are still in the ground and that they haven't been disturbed.
Well, Step One is to determine that they exist and that they haven't been disturbed. Being able to find them using their coordinates on a standard projection solves that very easily.
Step Two is to determine that they haven't been disturbed. I assume that part of that is confirming that the coordinate grid they mark on the ground isn't rotated with respect to the supposed projection grid. In other words, you want to verify that a surveyor can occupy a control point and locate things by survey ties computed in the supposed projection of the control coordinates without having to deal with the fact that the control network is on some skewed grid.
Having them all published on a known projection means that you shouldn't have to worry about solving a rotation between your new system and the original system. It also means that subsequent surveyors will be much more likely to be on the same grid.
Having them all published on a known projection means that you shouldn't have to worry about solving a rotation between your new system and the original system. It also means that subsequent surveyors will be much more likely to be on the same grid.
I'm not expecting to find any rotation. They were occupied and adjusted using GPS.
Even though it was quite some time ago they will unlikely be accurate. There has been activity in the area and some may have been disturbed or removed. If so my only task it to note which ones and move on from there. Only points I accept will show on my drawings.
Leon.
Where I'm at, the scale factors are so close to one, it's really really easy to put it on the grid, stay on the grid, and not have issues with the surface.
Leon.
Maybe so. At 31 47 45.28N & 95 09 02.15W 160M (525ft) I get your scale factor would be 1.000020896. That's about 0.02ft per 1000 feet or 0.11ft/mile. Meridian convergence angle 2 40'10" (almost 3 degrees).
Kent's really on the right track here for entering a list of coordinates as the first step. Given a list, I'd be OCR'ing the list to create an ascii file in a csv format. Set up your drawing zone for the designated state plane system and import the coordinates. Use CAD to connect the dots, then label the lines with the b/d. A quick comparison of the b/d labels with the drawing (or description) labels will expose whether the drawing has a cardinal rotation or any grid or elevation factor applied.
If the drawing is other than SPC Grid b/d, then you should be able to easily compute the bearing rotation or the scale factor by comparing the labels between the two. I'd then set up the transformation parameters in my drawing to account for the rotation & scale, re-import the point list, reconnect the dots, and re-label the lines. A final comparison of the line work labels will give you a final check.
You've just worked backward from the product data and reconstructed the LDP metadata they used to produce it. You can now work on their system in your drawing and can produce a list of SPC's when you're done.
JBS