I haven't read any responses but that method is fairly common. DOT's like to use geo-referenced imagery which this method allows except the farther away you get from the scale point the imagery starts to get off noticeably depending on the map scale. Although the coordinates look similar to state plane DOT's don't want you to truncate because the imagery won't overlay properly. This method works well in an east - west direction for Lambert and in the north south direction for Mercator. Conversely it falls apart faster at 90 degrees to the direction central axis of the projection. I've seen a few ugly results by going to far at 90 degrees... worse distortion than leaving it at grid. Some DOT's have quit fighting the scale factor game. It makes sense. click here
TxDot uses the method mentioned by others of scaling from 0,0 by a combined surface adjustment factor. The approved factor for each county is published if you can find someone that knows what to look for. I keep a copy on my DC and my computer.
I have two files in my data collector, one for the Texas central zone, the other for the north central zone. I can download a set of plans before I leave out. Stop by Office Depot and print the plans. Get on the future jobsite. In Survey Controller I open whichever one of the two files for the correct zone. I then start a new job and all the parameters from the generic zone file goes with it. change the surface adjustment to what it says on the plans or the published value for that county. Key in the control. Start my rover using waas correction and start looking for control.
When I find the first one I set my base on it and check in on several others. By noon I'm ready to start staking.
Once I had a job that used a surface adjustment factor specific to that job from a control network done thru a 4 county corridor. I called the District Surveyor and he gave me the correct factor.
I'm currently working on a job that has GPS control in State Plane and traverse control in between that is adjusted state plane. I didn't even try explaining it to the Engineer, it just goes over their head.
Some of the jobs are actually State Plane, one we are working on has the following note:
>Bearings are based on NAD 83 Datum. Texas State Plane Coordinate System. North Central Zone. All distances and bearings shown are surface values and may be converted to grid by multiplying a scale factor of 1.00000
In TxDot's defense, this one was done by a consultant engineer. I wonder if they multiplied ALL the bearings by 1 before they put them on the plans. :-O
We use machine control on all the jobs except rehabs. Never had a problem. The difference in SPC and scaled coordinates in most counties is around a 1000 feet so it would be hard to not see an error. I usually log a file the first day and send it to opus just for a check.
In my opinion, their method works well if an engineer doesn't screw it up before it gets to me. I know that a lot of surveyors cringe at the thought of scaling SP coordinates but in all honesty, TxDot doesn't do this for surveyors.
James
I used not to like doing it that way, but these control points have been done that way since at least the mid 70's.
Once computer programs became available it became clear that there are advantages to doing it this way...
It's so simple to import an ortho, or quad or bing map if your coordinates are organized with the DOT DAF system.
All the design build companies are so used to it it's second nature to everyone.
Exactly!!!
I never assume I have state coordinates.
Unless I have Metadata telling me coordinates are state coordinates I'm going to assume they aren't cause why would anyone work in them.
Squirt Guns?
Reflectorless I would do. If it was first man to set up a T3 and turn 10 sets I'd just relinquish the title to Dave.
Now wait just a minute!
Get off my lawn.
Lol. I forgot about your copyright on Curmudgeon. Begging your pardon.
Now wait just a minute!
That's OK - in fact how about considering me your mentor and your title can be "Curmudgeon in Training".
Now wait just a minute!
:good:
Norman you are hilarious. That was the best answer.
Tell the DOT Engineer to pound dirt and do it the way a Surveyor would do it.