laying out bridges using SPC grid coordiantes

You need know the direction of your scale factor. Is that scale factor ground to grid? If yes, then your ground distance is longer than the grid distance. The Tappan Zee was built on grid coordinates, that was 3.2 miles long. The temperature correction for the prefab steel will likely be more than the combined scale factor.

I think they are grid, MD SPC nad 83 based. How would one go about negating this scaling being a problem? I thought of scaling my cad drawing the same value the opposite direction so when the collector does its thing it cancels out. Any advice would be much appreciated

You need to set your cad drawing up so that you're working in the right zone.

How is the bridge situated on the grid. NJ is a transverse Mercator.

A north/south orientation will have a different at both ends vs an East/West.

One orientation may deal more with convergence.

Not sure about 1000 ft though. I'd work with grid to ground regardless. It makes good practice and you will be right.

Thing one is this: Always work in project coordinates. Make your measuring machine work in the project system. With your?ÿtotal station, you can have it apply the scale factor to every ground measurement you make so that it is reduced to the project system.

I know of a large bridge replacement project in NJ where the contractor's surveyor could not deal with the grid to ground conversions and had a hard time design with the whole concept.

The contractor reneged on the contract and paid 10% back to the NJDOT for non-performance.

The 10% was about $9 million!!!!

On another the design engineer did not understand the grid to ground conversion and the first steel girder dropped between abutments!

Ouch!

Are you laying out points for the bridge with GPS? If so I would be more concerned with the GPS error(s) than the scale factor error. If it were me, I would use the ground values for the control and use only a total station and half of your head aches will go away.

If the scale factor is properly done then the point you are staking 1000' away should be 1000.00' away.

much like the?ÿright of way I did in state plane that was to be 200' wide, I had to show it on the plat as 199.86' wide (SP)?ÿeven though it was 200.00' wide.

The fact of the matter is that on any bridge I have done over the years, it can be moved a few inches one way or the other and not cause any problems. I have never done any long bridges or any interchanges. Once you pin down where it will be, then use that method/control throughout the job.

I am not advocating just haphazardly marking the bridge location. I would mark the two centerline points (assuming it is a straight bridge) that are far enough back from the abutments to stay put for the duration of the job. Then I would set on one and shoot the other to get an accurate distance (NOT with the dc)?ÿ and adjust the station of each point to fit the distance. All layout would be done by station and offset from those two points. If you wish to use the data collector, you can setup a separate job file with the North being the station and the East being left/right of centerline. No SPC, no scaling.

I would also set two intervisible points off location from the two centerline points as a backup plan. Preferably one close to each abutment.

If this is an interchange, then don't take advice from a country surveyor that hates even driving thru an interchange, much less staking one.

James

Start in the middle for 0.03 each way. ?ÿGGGRRRIIINNN.

That setup might have some stability issues...

If I were in your position I would measure the grid distance between your two abutments in project design files (that I assume are state plane).?ÿ If they are 1000.00' apart, that means the bridge was laid into the design without regard for state plane grid/ground.?ÿ In that case, I'd pick a point from middle of the bridge and scale the coordinates for the abutments from there.?ÿ Then I'd check the distance again.?ÿ Most software should be able to show grid vs. ground, and you want ground to be 1000.00.?ÿ ?ÿ?ÿ

Don't over think, if you do, you are bound to create error. Just apply the combined scale factor in your data controller. It will apply the CSF to all EDM distance and reduce the COGO to grid coordinates. The DOT's control points and contract plans are on SPC (GRID), therefore you should stay on the grid. Don't mix apples and oranges.

You can verify by setting up on the existing control and shooting distances to other baseline points.

?ÿ

What controller and software are you using. Most software today will compute the CSF at each control?ÿpoint, and apply accordingly when setup to use a SPC projection. This goes for both EDM and GNSS.

On DOT projects you should never, never, never need to use a localization or calibration. Always use the SPC projection. If you think a localization is needed, then that is your first clue something is wrong.

What ever you do; you need to have project coordinates on known, undisturbed points, and check a distance between them, using the total station and data collector designated for this project.

?ÿ

An ounce of prevention...

The answer is simple and lies in understanding what you have in the control and in the design datum, then getting some settings right on your equipment. It's no big deal. But it is true that not every surveyor, even the licensed ones, understands how to work on the grid. I'd suggest that you bring in a professional who does to help you out with the set up. It will be money well spent.

And hope the bridge steel fabricator knows what SPC with CSF means, too, or somebody does the translation to true length at specified temperature for them.

Even if you do it right you will get sued when the parts don't fit.

In the absence of evidence to the contrary it would be fair to assume that everyone from the design engineer to the steel fabricator will not account for the scale factor so you will need to convert your grid coordinates to ground and work in that. (as described above). The difference can be lost in the road construction either side of the bridge.

If it's a series of bridges and if there are other contracts/surveyors then it could be more complicated.

Make sure you issue queries or emails to that effect and be prepared to have to explain the whole thing

That said, 5.7mm in 100m is bugger all and unlikely to ever be noticed.

Round here it's 40mm in every 100m.

Interesting post and replies. I've been around bridge engineers for a long time. The last thing they would do is design a bridge and quantify bridge parts on any scale but 1.?ÿ I would think long and hard before staking out a bridge on any other scale.?ÿ As others have pointed out your total length difference between grid and ground is 3/4".?ÿ Where you could get off is where the center of the bridge is supposed to be without considering the scale from a distant control point. After you figure out where the bridge belongs any scale other than 1 is fools play I say.?ÿ

Posted by: squowse

In the absence of evidence to the contrary it would be fair to assume that everyone from the design engineer to the steel fabricator will not account for the scale factor so you will need to convert your grid coordinates to ground and work in that. (as described above). The difference can be lost in the road construction either side of the bridge.

If it's a series of bridges and if there are other contracts/surveyors then it could be more complicated.

Make sure you issue queries or emails to that effect and be prepared to have to explain the whole thing

That said, 5.7mm in 100m is bugger all and unlikely to ever be noticed.

Round here it's 40mm in every 100m.

Sorry, to be clear you don't need to "convert" your setting-out co-ordinates, just use them as ground co-ordinates so they tie up with the length of the bridge.

You will need to convert your control points by from scaling from the centre of the bridge (or one end depending on where you want to lose the discrepancy).

Use these control point co-ordinates for setting out the bridge, the distances between them should have a scale factor of 1.

?ÿ