hello guys, i am working on a project where we have to build a 300m span bridge over a river in Ghana. We are working in the WGS84 zone 31N coordinate system. I want to know whether converting from grid to ground coordinates apply in this case when setting out
My opinion is always, always, always be in ground for bridge work.
UTM projections can have up to 1:2500 scale factor from grid to ground, and the combined factor could be more (or less) depending on where you are in the zone, and depending on the elevations you are working at. 1:2500 translates to more than a decimeter of error across 300m.
When building real things it is a mistake if the real thing is not designed using ground dimensions (coordinates) or laid out using ground dimensions (coordinates) No one should design a scale model, layout a scale model, nor build a scale model.
@lurker thank you
@rover83 thank you very much
@rover83 how do you get the combined factor to convert grid to ground
I also like to work in a world where 1=1 less and except curvature correction.?ÿ
Know your combined scaled factor, especially if the bridge design was done in Grid coordinates, which I highly doubt it. It's common practice, at least where I live, to design everything in ground coordinates. My suggestion, make sure your coordinates are ground coordinates, or localized coordinates specific to the site project area.
When this section of road was built back in the 80's the contract was in two sections - north and south of the junction. The designs for each were based on Grid rather than Ground with the start points at the far end of each contract. When they "met" here the alignment was around 2ft. out and the level difference was 1ft. The alignment was fudged between the two points where the slip roads entered the main carriageways, but for the first 5 years or so it was very obvious to the eye. After 30 years and full of traffic it is no longer apparent.
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https://www.google.co.uk/maps/ @55.0041742,-1.6704846,3a,75y,23.82h,89.72t/data=!3m6!1e1!3m4!1s_oKUkp2eYI5iO_CJdvgBWg!2e0!7i16384!8i8192
Also watch out for designs that use 1 UTM coordinate AND proceed with no scale factor. With just a north arrow and no other information. It may only look like UTM.?ÿ
Find out what the design is truly in, and what they did.?ÿ
Broken link.?ÿ Try this one.?ÿ If it works, the US folks should take a deep breath before looking at the street view:
@55.0041742,-1.6704846,3a,75y,23.82h,89.72t/data=!3m6!1e1!3m4!1s_oKUkp2eYI5iO_CJdvgBWg!2e0!7i16384!8i8192" target="true" rel="noopener"> https://www.google.co.uk/maps/ @55.0041742,-1.6704846,3a,75y,23.82h,89.72t/data=!3m6!1e1!3m4!1s_oKUkp2eYI5iO_CJdvgBWg!2e0!7i16384!8i8192
map:
@55.0090501,-1.6896007,13z/data=!4m5!3m4!1s0x48795f8e6e2c6e89:0x1c23eeb423741284!8m2!3d55.0041481!4d-1.6704578" target="true" rel="noopener"> https://www.google.co.uk/maps/place/A1,+United+Kingdom/ @55.0090501,-1.6896007,13z/data=!4m5!3m4!1s0x48795f8e6e2c6e89:0x1c23eeb423741284!8m2!3d55.0041481!4d-1.6704578
Gird or ground aside - If the bridge was designed 300 m in length I'd make sure it was staked out 300 m in length as measured with a total station using a scale factor of 1.?ÿ ?ÿ
Working on one over the Cumberland River now. Part of a road relocation, everything is spc, stations for the 9 piers are based on the road alignment stations. So I established the beginning station using spc, and set a site on the other side past the bridge end using spc. Created another file at 1:1in Access using 0,0 as beginning station coordinates and used span lengths and due north to establish the pier locations and offsets. I use that file and my offsets for any bridge layout.?ÿ
In New York all bridges are designed and built on SPC. Not sure why others feel this is so difficult. Use the proper CSF and all works fine. The Tappan Zee Bridge is 3.2 miles long, no problems. Perhaps it is more difficult at higher elevation or with a large change in grade across the project.
It could be done either way.?ÿ Just be absolutely sure everybody on the project does it the same way.
But I don't suppose you can order girders dimensioned in SPC (SPC at what location?). Somebody has to do the conversion somewhere along the flow from survey to design to materials.
yeah a 3000-4000 grid feet span design.?ÿ
I??ve seen the over all alignment in SPC, but the bridge had a separate plan with a ??work point?? and centerline called out and dimensions from there. No ambiguity.?ÿ
Obviously bridges are built using ground coordinates.?ÿ Sad when prestressed beams show up and they're two inches too short because some yahoo surveyor used SPC to establish the abutments and piers.?ÿ Worse is steel construction where 1/2 inch is a problem.?ÿ Bridge construction is purely local inches/feet dimensions and I use Total Station/optical levels to keep my errors down to a few hundredths, no mumbo jumbo GPS SPC bullshit.?ÿ Learned early on to establish tight ground?ÿ local control stations out of the construction prism so if a pier formwork slumps and the contractor claims it was bad surveying I've got the proof to absolve my work.
It's the same for all civil engineering works, tunneling, large buildings, industrial sites; show up with optical gear; local positioning is paramount and true Lat-Lon means nothing.
All of the bridge shop drawings had CSF on every sheet.
The expansion of steel due to temperature exceeds most CSF.
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Obviously if you can't work on grid you are not much of a Surveyor.?ÿ
The expansion of steel due to temperature exceeds most CSF.
~6.5 ppm/?ø F, so for say 30?ø F about 200 ppm or?ÿ 1:5000.?ÿ
SPC are intended to have distortion less than 1:10,000 or 100 ppm.?ÿ Elevation factor of 0.9999 (100 ppm) comes with a change of about 2000 ft.
So in areas of moderately high elevation away from the central line of the projection, the thermal and CSF may be comparable.
Why SPC and not an Low Distortion Projection, avoiding further need to worry about it?
All of the bridge shop drawings had CSF on every sheet.
The expansion of steel due to temperature exceeds most CSF.
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Obviously if you can't work on grid you are not much of a Surveyor.
If you're using a CSF, you're not working in grid any more.
And that's just needlessly convoluted.?ÿThere's just no need to make every designer and fabricator put a CSF on their drawings. Develop a ground coordinate system with a defined relationship to SPC, and then design and work in it. Makes it easier on everyone, including the surveyor.
It's not about whether the surveyor is able to work in grid or ground; the rest of the design/build industry does not, and shouldn't need to, concern themselves with doing so. Of all the things that are critical to a structural project, being able to locate it in SPC is not among them.
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