TDOT GPS Control Points

Makes me wonder if it is one of those "divide by a combined scale factor" kind of thing?

Stephen,

You are correct, TDOT has their own "system", if my memory serves me correctly. The origin is set up in northwest TN.

I ran into this many years ago while working with a former employer. We were working on a TDOT project in the Chattanooga area. Your experience is very similar to mine.

TDOT owns and maintains the HARN points which I know have correct published state plane coordinates. Maybe they do something on a project by project basis that causes their coordinates to be off from standard Lambert grid coordinates.

I'm going to the TAPS conference later in the week. If I run into anyone from TDOT there, I will inquire about the method to their madness.

First off, the Tennessee HARN points are not even accurate enough for NGS to accept.

Now on to the important question. The TDOT coordinates are not State Plane NAD 83 Coordinates. Each project has a customized scale factor, which is published on the plans and on the control point sheets. They dumbed it down so that the contractors would measure a distance and it would equal the published distance between monuments.

It looks goofy as hell when you first run in to it. But after you get used to it, it's no big deal.

Thanks for the explanation. I've shot their points numerous times to help lock down highway right of ways but this is the first time I've had a reason to compare them to other control.

I misspoke when I wrote HARN in my previous post. I meant to refer to the TGRN system established by TDOT that has 60 stations evenly spaced across the state which do have NGS data sheets. I definitely understand the quality or lack thereof of the GPS control points that TDOT sets for project control.

Try scaling the DOT points using the correct scale factor from 0,0. In Ky, they give SPC and project control coordinates. The project control coordinates are computed using the project from the central point of a project. To get SPC, you divide the project coordinates by the project datum.

By HARN points, I meant HARN points, not TDOT reference stations and NOT TDOT project control monuments.

Also, here is something else to keep in mind. When retracing TDOT right-of-way, especially recent projects, pay no attention to the right-of-way monuments. The right-of-way corners are where the coordinates say they are, not where the concrete is in the ground. I know, I know, that goes against the law. However, that is the opinion of TDOT.

>"Each project has a customized scale factor, which is published on the plans and on the control point sheets."

That's a valid way to simplify life for contractors, EXCEPT that it is absolutely stupid to use the same number of digits as SPC, resulting in this type of confusion. It would be trivially easy for them to subtract some offset from the modified values so that they look different and won't be confused.

I understand that. However, nobody that isn't using the monuments for highway construction have any business using the monument coordinates. These monuments are not part of some sort of state sponsored control network. They are for building highways, period.

The problem in this thread is the OP attempted to use the monuments for a purpose that was not only not intended, but advised against by every source that contained the monument coordinates. If you call TDOT, they won't even give you the monument coordinates. You have to go to the office and write them down from the plans.

For the record, I obtained the data sheets from the Microsoft Access database that TDOT has available at their website. I also acknowledged their disclaimer in my original post. I was not trying to start a fight or throw rocks at TDOT, just trying to educate myself as to why there is a difference between their coordinates and SPC.

I also appreciate the info concerning TDOT's position with regard to concrete monuments vs. coordinate positions of the ROW. This particular project has 2500 feet of frontage on the state highway with no concrete monuments remaining. I'm working on an as-built of the existing entrance from the highway into the property which is located about halfway along the frontage. I had planned to show the ROW line in the project area per the TDOT control points and the coordinates shown for the ROW line. It puts my mind at ease knowing that TDOT would use the same method.

I agree with Tommy.

We have had the same thing here in New York, mostly with older projects.

The highway ROW is determined in relation to a design baseline (which may or may not be the highway centerline).

The ROW angle points, PC's PT's etc., are given in terms of station and offset along the baseline, and when a description of the taking is written, that is how it is referenced.

The monuments in the field are not to be assumed correct.

Up until a few years ago, the contractors who build the highways were never made to certify the placement of monuments....I've personally observed laborers going out with a cloth tape to do the setting.

Nowadays, certification by an L.S. is part of the standard NYSDOT contract.

Cowboy

Unfortunately, I've seen ways around that cerification too. The certification stated that the surveyor "staked" the monuments in the correct location. Then the laborers come along, pull up the stake, stick an auger in the ground and place a R/W monument. If there is a rock or root in place just move it over a foot or two, nobody can tell the difference. Right?

Andy

Andy

Here in New York, they make it an "as-built" survey......after the monuments have been placed.

We have submitted many tables showing "deviations" (mostly pretty minor, i.e. the proverbial 0.04 foot) which are usually noted and filed away.