Low Distortion Projections and the law

I have the same requirement with filings in the county I live in. Because of that I do everything here in state plane. The central meridian does pass through the county so bearings are generally "fairly" close to true north, so it lessons the need to set up a LDP. However, all my plat distances are shown as ground with a scale factor which is stated on the plat. If you need to show the relationship between state plane and your LDP plat you will also need to figure out that number. What would be the scale factor to convert your ground distances to state plane distances. Also state your origin lat and long for your LDP, the type of LDP, what value you used to get it to ground, the rotation between state plane and your LDP either along its central meridian or a line on your plat. It's also important to show coordinate values in your state plane on your plat. In my case there is a requirement to make a section corner tie and I show the coordinate there with a plane coordinate, lat, long, show that's it's NAD83 and how it was derived (tied to CORS points, epoch, ect.). Of course maybe for you another point will work, say your POB. I'd also show a LDP bearing and a state plane bearing along a line on the plat.

> ... NC has a law requiring me to tie to State Plane...
If I'm doing something merely to comply with some law, and for no other reason, I'm going to look for the easiest, quickest way to get past it. It seems to me that giving SP on a couple monuments and the convergence angle would be the way to go here.

OK - I've got to ask

Why are you even bothering with any kind of a special projection? I looked up combined scale factor corrections in the Wilmington area and if I'm reading the data sheets correctly the magnitude of the corrections is on the order of 1 part in 100,000 - or less. So, unless you are surveying one heck of a big area you wouldn't be able to quantify the differences between GPS measurements and your total station measurements.

To put it in real terms, the difference in a measurements distance of 2 miles would be about 0.10'. There are darn few of us that would want to bet we are that good with all our procedures to insure that.

Maybe I'm showing my stupidity here and if that is the case, have at me and educate me.

I may be mistaken about this but couldn't you look at the WGS84 coordinates and use CORPSCON to convert them to State Plane?

State Plane Coordinates are defined in NAD 83 not WGS 84.

We're not under any legal requirement to put State Plane coordinates on our plats in Texas (except for local jurisdiction regarding new subdivisions). But we've been putting State Plane on almost every job we've done since 2001. I say that to convey this: I don't know if this will satisfy your State's requirements, but it might.

We just completed a job here in town where I created one of my LDP's. The CAD file, the bearings, the distances - all in LDP (Kilgore Coordinate System). I then labeled all of the boundary corners and reference monuments with State Plane Coordinates. And this is the metadata that went with the plat and description:

Bearings related to a local grid having an origin of North Latitude: 32°23' and West Longitude: 94°52'. To convert reported bearings to bearings related to Grid North for the Texas Coordinate System of 1983, North Central Zone, rotate reported bearings counter-clockwise by the mapping angle of 1°58'54".
Distances are expressed in US Survey Feet as measured horizontally along the surface of the Earth. To convert reported horizontal/surface distances to distances measured along the Grid for the Texas Coordinate System of 1983, North Central Zone, multiply reported distances by the project combined scale factor of 0.999928.
Cartesian Coordinates are expressed in US Survey Feet as determined by GPS observations made on site and are related to the Texas Coordinate System of 1983, North Central Zone, HARN - 1993 Adjustment.

If you inverse two State Plane Coordinates listed on the plat, rotate the resulting bearing clockwise by 01°58'54" and divide the resultant distance by 0.999928 you will get the same bearing and distance shown on the plat. Presently, I don't list any coordinates in my LDP on the plat.

Just fyi - we used HARN instead of the more superior CORS96 or NAD83_2011 because the City of Kilgore has a monumented network based on HARN. So we keep our city work in HARN.

OK - I've got to ask

1 part in 100,000. That would be coordinate heaven to me, I would stay with state plane in that situation also.

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Well done Shawn.

And of course the intent of the law is to provide a geodetic coordinate system for the survey so those who wish to know where in the world it is. LDP is a geodetic coordinate system so the intent of the law is there. The provider needs to publish the geodetic parameters of the system but if that is done conversion to state plane, UTM or any other system is only a step away.

Just fyi - we used HARN instead of the more superior CORS96 or NAD83_2011 because the City of Kilgore has a monumented network based on HARN. So we keep our city work in HARN.

I think the older users of GPS mostly started laying out control using HARN monuments and the 93 epoch. Of course this caused a lot of data to be accumulated in that coordinate system.

I've got the same sets of data here, the city control on NAD83(93) adjusted from a local HARN points. It works well, but.....

It is amazing how surveyors can't get it though their heads that OPUS isn't going to match. There is abundant available control on that system, they just won't use it because they have something "better".

Oh well!!

We have an offset we've determined locally from CORS96 and NAD83_2011 to our local HARN. It's about a tenth of a foot horizontally. Enough to notice when you're checking a HARN derived project to OPUS.

I've been thinking about doing a reobservation of the City monuments (those that have survived) into NAD83_2011. There are only about 30 of them.

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thank you

>It is amazing how surveyors can't get it though their heads that OPUS isn't going to match.
A great many surveyors using GPS - I think more than half - simply do not understand geodesy on even the most basic level. The salesman helps them get the settings in their dc's right for their area, and they accept what comes out.

> I've been thinking about doing a reobservation of the City monuments into NAD83_2011. There are only about 30 of them.
If you had access to the original network observations in, say, StarNet format it would simply be a matter of feeding in "held" coordinates in the new datum and re running the adjustment. No need to reobserve every time the NGS readjusts. If the held coordinates in your network were CORS their would not even be a need to return to the field.

Perhaps by forensically deriving the x,y,z cartesian coordinates for each monument in NAD83(93), then figuring out the delta x,y,z between the various adjacent monuments, you could construct a psuedo "observation" data file.

Good point, Norman. I've been thinking about that approach. I have all of the vectors on paper (including the covariance matrices). The company that did the original did really good work - particularly for the time, compared to what others were doing. There were four or five main points derived from the HARN and then the remaining 25 or so were networked off of those four or five. So, I could simply reobserve the four or five and readjust. I may do that yet, but I'll have to manually enter all of the vector data (OCR might work).

But reobserving would have the benefit of assessing the health and status of the network.

It would all be pro bono, so I can do whatever I want, I suppose. Mostly for the fun of it.

> State Plane Coordinates are defined in NAD 83 not WGS 84.

My understanding is that over a small area there isn't that much of a difference between NAD83 (1986) and WGS84 (original). However I'm looking into it a little further now. You have piqued my curiosity!

For much of the US the difference is on the order of a meter. It isn't going to matter much whether your area is a lot or a county, it's pretty close to a simple shift, unless the local NAD83 as defined by the monuments has a kink in it.

NAD 83 and WGS 84

The "official" transformation between NAD 83 (1986) and WGS 84 (original) is 0 each in X,Y,Z (as defined by DoD in TR8350.2) with an uncertainty of +/- 2 m in each component. Today the difference between NAD 83 (2011) and WGS 84 (G1674) in the conterminous U.S. is about 1.5 m (3-D) +/- 0.2 m.

Since the only way to get the original WGS 84 coordinates was by autonomous point positioning then the uncertainty was already in the range of 10-100m depending on how long you let the receiver cook.