Effects of grid and ground coordinates on site
This might sound amateurish but I am a surveyor new to the field and new to this site and i’m hoping someone can clear something up for me…
I work in the uk and I cant quite wrap my head around the grid v ground debate. I understand the differences between the two and why they are both required for map projections etc. but i’m struggling to understand which is the best to use.
I know it depends on the size of the site but we often require our surveys to be based on OSTN15 coordinates (the UK coord system), so I tend to use a GPS rover in a lot of surveys and find doing everything on grid (with a scale factor) is much simpler, when using GPS. unless it is a small site and use only a total station where i will use a SF1. We also state that its been provided as grid coordinates and the scale factor so there is no confusion.
However (for example), if a house was drawn to be 10m long on plan (based on my grid survey) and then it was built on site to measure 10m long with a tape, if I was to resurvey the house, would it be exactly the correct size? (hypothetically, minus building tolerances and without setting it out, etc etc)
Or if everything was based on a grid with a SF, and then structures were to be set out by a surveyor on grid and then measured with a tape, would the distances/locations be different? If so, surely some sites surveyed and provided on grid are actually bigger or smaller than what they appear to be?
We have a large survey for a proposed pipe route coming up about 10k long, now i know we need to take in to consideration the curvature of the earth for this, but how would this affect the amount of pipe required? if that makes sense? will a survey in grid taking into account the curvature of the earth provide the correct amount of pipe or will it be more/less than if i were to use a SF1?
Also, when scaling factors are automatically applied when using GPS, where is the origin of the scale? i’m assuming its the zone origin? i.e N49 W2 as it is in the UK?
Apologies for the ignorance and the what some people may find, stupid questions, but I can’t find much online that puts it in layman’s terms or relates it specifically to topographical surveying/on site construction.
Thanks in advance!
- 8 Replies
Grid is the answer with designed distortion low enough to make the difference between the two irrelevant for the purposes of the project.
I think in terms of PPM. How far from an actual measured distance is the grid distance. My projects range from 100ppm to almost 1000ppm, grid vs ground.
It’s important to know that value before deciding to adjust for it.
For pipeline projects that I’ve been involved with, they’ve always wanted ground (I’m not a pipeline surveyor). I even have one that re-adjusts every 10k.
Take your 10k on grid multiply it by your combined scale factor ( grid factor and elevation factor) and you will have your length of pipe. But you will still need to account for curvature. If you have grid coords that inverse 1000′ between them and you occupy those points with your total station, you will shoot a distance longer than 1000′. If you stake out a 1000′ long warehouse using grid coords and they build it to your stakes the warehouse will be shorter than 1000′. This all assumes that you are located in an area where the scale factor from grid to ground is greater than 1.00.
Best idea is to develop a grid that fits the project, as Norm mentioned. But lots and lots of pipeline projects are kept in large-scale grid systems such as national and state projections.
But even if there’s some distortion across the project, they’re not going to requisition exactly 10k of pipe. They’re not going to care that there’s an extra meter or two versus what is shown on the planset. At least I never heard anyone complain about it. It’s a tiny fraction of a tiny fraction of a percentage point of the project cost. They mainly care about keeping it inside the designated ROW and at the require cover depth.
“…people will come to love their oppression, to adore the technologies that undo their capacities to think.” -Neil PostmanThanks for all of the replies so far – in regards to the comment about the amount of pipe, although we do have this project upcoming it??s meant more of a hypothetical to question the difference between grid/scale factor and ground/SF1
Road design engineers have told me they have a 20000 ppm error budget for design quantities. So 20 ppm or even 100 ppm in grid error needs to be put in proper perspective based on project error budgets.
Hello
UK based as well.
The main problem that I find is lack of clear notation on survey drawings about whether a survey has been drawn to grid OS with appropriate scale factor or plane co-ordinates centralised to OS grid a single point (so well done for making note of this on your drawings).
The key I think is leaving sufficient safe control stations around the site that future surveyors and site engineers can check and see what is going on. Doing everything with GPS is not a license to leave no control stations.
Personally for sites smaller than 1km x 1km I would say remove the scale factor and centralise to OS at a single point but that is mainly because I know how little site engineers over here seem to know about scale factors and it will avoid headaches for them further down the road (literally!). But what do I know!
Best idea is to develop a grid that fits the project, as Norm mentioned. But lots and lots of pipeline projects are kept in large-scale grid systems such as national and state projections.
But even if there’s some distortion across the project, they’re not going to requisition exactly 10k of pipe. They’re not going to care that there’s an extra meter or two versus what is shown on the planset. At least I never heard anyone complain about it. It’s a tiny fraction of a tiny fraction of a percentage point of the project cost. They mainly care about keeping it inside the designated ROW and at the require cover depth.
its pipeline, so couldnt you just keep tying vertical BM by location, transfer by levelling, and carry the elevations between known BM and just blast out XY with gps thats easy peasy fast and greasy forthe majority of the work?
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