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I am a civil designer with limited survey experience. I am learning about the concepts behind geographic coordinate projections. I understand to change from a 3d geographic coordinate to a 2d map, that a projection is required. However, it seems to me that to change survey data from an ellipsoid grid to a ground survey, also involves a projection. Is my understanding correct and is the projection handled the same way ?
This Content Originally Published by a land surveyor to Land Surveyors United Network
Your question is a bit confusing as there as I am not sure what "ellipsoid grid" is, never heard that term before. What I believe you are referring to is state plane grid coordinates which an ellipsoid is part of the calculation that converts geodetic coordinates (lat/long) to state plane coordinates (a.k.a. a flat surface plane). When you project from geodetic to state plan there is grid scale factor that is involved in addition to elevation factor. Be advised this is a projection to a flat surface and thus it is not actual ground coordinates.
To convert your coordinates from grid to ground you need to apply the combined scale factor = elevation factor x grid scale factor. https://rplstoday.com/community/threads/grid-to-ground-combined-sca...
Best of Luck.
Thanks a million for your feedback. I realize my survey understanding is sketchy and experience even more, thank you for sharing your insight. It sounds like my first misunderstanding was the terminology of "the grid". Apparently that only applies to a projected map grid, not to the lat-long grid on the ellipsoid datum. Also, it appears the survey work-flow produces a projected map ("on the grid") that can get scaled to a ground survey but still a projected map. So the work flow is not likely to take the 3d geographic lat-long and project directly to the ground without first being projected to the "grid" on a map ?.
Every scenario is different so I am not sure what exactly your project is. If its just a small project 10 acres or so and you have point data that is in state plane then I would bother messing with it as it's not going to adjust the horizontal distances all that much a few tenths of a foot. If you have a large project like linear roadway then that's a different approach.
If you get survey point data that's stated its in state plane coordinates then technically you should always apply a combined factor to project it to ground because you want to work with ground data. Whom ever sends you that data I would have them put it in ground coordinates but the coordinates should be truncated by adding a 9 to all the points or something to that affect that way you know its ground data not state plane. If you don't you will get confused to what point data is what because the coordinates will be near the same perhaps a 6" to a few feet different (x,y speaking).
That's my two cents as a PE/LS
Is it possible to scale a 3d geographic coordinate to a "ground" survey without projecting the coordinate to a flat map ?
When performing a topographic survey is not projected anything, is only measured on the ground, is considered land as flat (for small areas), projection is a faithful form of land representation and each country adopts the system That will be used within its borders, using the one that most suits your needs, for example in Colombia my country uses a projection that retains the angles but distorts the distances, there is no projection system that retains angles or distances to The time... :)