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laying out bridges using SPC grid coordiantes  

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Junious Mays
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I have a construction project that I am woking on, specifically a bridge that is over 1000' from abutment to abutment. The provided control is maryland SPC nad 83 and the control report states that a combined factor of .999943 is to be applied. when applying a scale factor in the data collector from my understanding evrything is scaled even stakeout. My concern is that if i stake out a point that is supposed to be 1000' away is it going to actually be 999.943' or 1000.057'. Are my concerns unfounded or am I on the right track. I know most folks would not worry about .06  but we have prefab steel and we want to shoot for perfect upfront knowing there will be some error introduced. 

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linebender
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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.  I would think long and hard before staking out a bridge on any other scale.  As others have pointed out your total length difference between grid and ground is 3/4".  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. 

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leegreen
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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.

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Junious Mays
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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

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Steven Metelski
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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.

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

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Chuck S
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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!

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