Land Surveyor Forum

Replies

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  CHARLIE B. AYCOCK, III January 8, 2017 at 3:26pm

  Before you can define an elevation, you have to specify what you are talking about & you have to define the datum you are using. The elevations most often refereed to are orthometric or the vertical distance above or below the geoid measured along the plumb line. There are some 200 vertical datums in use today. It could also be just an assumed datum used to see which way the water will flow. The distance represented by a coordinate pair on the reference ellipsoid to the point on the surface of the earth is measured along a line normal to the ellipsoid & is called the ellipsoidal or geodetic height. Ellipsoidal heights are not all the same as reference ellipsoids or their origins can change.
  I quote from Jan Van Sickle's book, GPS for Land Surveyors, " The Purpose of the state plane coordinate system was to overcome some of the limitations of the horizontal plane datum when they are applied over large areas while avoiding the imposition of geodetic methods & calculations on local surveyors." It appears we are headed toward a global system based on GRS80 or something similar. NAVD 88 will soon be replaced by GRAV-D. Of course since the geoid is defined by gravity, it is bumpy & irregular like the surface of the earth & not nice & smooth like the ellipsoid. The problem is converting GPS heights to orthometric heights. Much progress has been made with geoid models & continues to be made.
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  CHARLIE B. AYCOCK, III January 7, 2017 at 12:10pm

  Well, the above is mostly true, especially when you are only concerned with practical field application. In this country, State Plane coordinates are generally based on either a Lambert or a Mercator projection. They are not flat but are designed so that errors associated with the curvature of the earth & the mapping angle will be minimal & generally errors will not exceed 1/10,000. These errors can be neglected for most survey work & it approximates the geoid model it is based on, as I understand it. Coordinates from a grid represent the distance from the origin on that grid. Quoting from NGS, "There is not a direct connection between GPS coordinates & a tangent plane survey", & "-- any tangent plane survey begins to fall apart at some distance from the origin point." The booklet "Fundamentals of State Plane Coordinates" was written back when I first used State Plane Coordinates, long before GPS existed. It is still valid. The updated version is called the NGS5 manual.
  I was talking to the guy that heads up our State Geodetic Survey, recently. We were discussing a book on GPS surveying & he said, "I have to read stuff like that several times before I understand it". That made me feel much better to know I'm not the only person that doesn't grasp everything the first time around. So, if you don't understand everything immediately, you are not by yourself. I only hope that one day I will know as much as I don't know.
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  David Andrés Herrera Arango January 6, 2017 at 9:07am

  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...  :)
• Land Surveyor

  John Wright January 5, 2017 at 1:30pm

  Chuck,

  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.
  • Land Surveyor

    Dean Burke > John Wright January 6, 2017 at 6:16am

    Chuck, what are the extents of the survey that you are working on? As John said- unless you are working on large areas or very long distances such as highways the difference will be negligible.
    Also check with the surveyor as some will include the settings in the survey, either within the instrument settings or as part of the post processing. Most site guys don't normally do this but it will be best if you confirm as you obviously do not want to adjust if not necessary.
    • Student Surveyor

      Chuck Turlington > Dean Burke January 6, 2017 at 7:16am

      Is it possible to scale a 3d geographic coordinate to a "ground" survey without projecting the coordinate to a flat map ?
  • Student Surveyor

    Chuck Turlington > John Wright January 5, 2017 at 1:55pm

    John,

    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 ?.
    • Land Surveyor

      John Wright > Chuck Turlington January 5, 2017 at 2:09pm

      Chuck,

      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).