It appears from prior discussions that there is no universal practice for plats, but the majority label with EDM distances, or convert from GNSS measurements to horizontal ground distance at average elevation, or use a fixed datum adjustment factor that is nearly the same thing.
I find it a bit strange that Iowa doesn't seem to specify, unless you can stretch "the horizontal plane" to mean "the horizontal plane at the surface," or even require labeling to tell you which is on a filed plat. The use of "plane" almost suggests SPC distances, but I doubt that was the intention.
355.7 Plats of Survey
10. Distance shall be shown in decimal feet in accordance with
the definition of the U. S. survey foot. Distance measurements shall
refer to the horizontal plane.
How about other jurisdictions, do they require ground, or a statement if not ground distance?
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This came to my attention because I'm interested in narrowing down search locations for any monuments (probably nonexistent) on my sister's property, and have a couple OPUS sessions on monuments of a nearby (not abutting) survey to extrapolate from. They are quite good enough for reconnaissance, but it bothers me that I miss the plat distance (around a half mile) by 0.225 ft if they are ground, but only 0.075 if they are SPC grid, and the plat has no statement of which is used.
The difference falls well within the allowed tolerance for rural surveys, so maybe he thought it wasn't necessary to state. The assumed bearing of his basis line appears to match SPC within a few seconds and convergence would be several minutes, so it seems likely he worked everything on grid, but didn't say so.
Subdivision Statutes date from the time of measuring with chains and steel tapes - obviously ground, although they 'could' be transformed to state plane grid distances. In California, the Public Resources Code specifies that the use of SPCs and grid distances on maps include minimum metadata for the purpose of locating monuments on the ground if necessary.
I interpret this to be a poorly worded requirement to display distances as horizontal and not slope. I don't think it has anything whatsoever to do with State Plane...
I agree, they are trying to say horizontal. I almost always show ground distances. A couple of time I have used the distances from state plane coordinates but I always say so on the plat. Without reading the specs, I don't think Illinois says either way.
Arkansas Standards of Practice for Property Boundary Surveys and Plats (revised May 21, 2009), Section 4.1.A.9, ÛÏNorth arrow with basis of direction. A statement shall be made to explain how direction was obtained, and should include document (book and page) references if based on deed or survey record bearings. When the basis of direction is derived from the Arkansas Coordinate System 1983 (geodetic or grid system), the convergence angle and, if the distances on the plat have been converted to ground, the Combination Adjustment Factor shall be shown with a notation specifying the location where the calculations were made.Û
I used to specify where the calculations were made as "the hood of the truck", but have now updated it to "in the cloud"...:beer:
DDSM
Dan B. Robison, post: 422013, member: 34 wrote: Arkansas Standards of Practice for Property Boundary Surveys and Plats (revised May 21, 2009), Section 4.1.A.9, ÛÏNorth arrow with basis of direction. A statement shall be made to explain how direction was obtained, and should include document (book and page) references if based on deed or survey record bearings. When the basis of direction is derived from the Arkansas Coordinate System 1983 (geodetic or grid system), the convergence angle and, if the distances on the plat have been converted to ground, the Combination Adjustment Factor shall be shown with a notation specifying the location where the calculations were made.Û
I used to specify where the calculations were made as "the hood of the truck", but have now updated it to "in the cloud"...:beer:
DDSM
Aren't the distances on a Property Boundary Survey always "ground"?
Dan B. Robison, post: 422013, member: 34 wrote: Arkansas Standards of Practice for Property Boundary Surveys and Plats (revised May 21, 2009), Section 4.1.A.9, ÛÏNorth arrow with basis of direction. A statement shall be made to explain how direction was obtained, and should include document (book and page) references if based on deed or survey record bearings. When the basis of direction is derived from the Arkansas Coordinate System 1983 (geodetic or grid system), the convergence angle and, if the distances on the plat have been converted to ground, the Combination Adjustment Factor shall be shown with a notation specifying the location where the calculations were made.Û
I used to specify where the calculations were made as "the hood of the truck", but have now updated it to "in the cloud"...:beer:
DDSM
That standard has so many issues; it's kinda like I know what they are trying to say and I just wish they would have said it.
"with a notation specifying the location where the calculations were made" That's a classic, a little like Bearings are based on GPS.
Also, you have 1/5000 + 0.26 ft tolerance in that state.
Steve
Bill93, post: 422003, member: 87 wrote:
I find it a bit strange that Iowa doesn't seem to specify, unless you can stretch "the horizontal plane" to mean "the horizontal plane at the surface," or even require labeling to tell you which is on a filed plat. The use of "plane" almost suggests SPC distances, but I doubt that was the intention.
If you look further down in 355 it defines the "plane" 355.16 Iowa plane coordinate system defined. I also have been told it was not the intent and it's certainly not practiced but words do have meaning. If everyone would use IaRCS as grid and ground there would be no confusion.
I can see that as a possible interpretation, but also doubt that was the intention, and note that 355.16 defines Iowa plane coordinate system" and not "plane."
I wonder if there is any legislative effort underway to define the Iowa Regional Coordinate System (IaRCS). For those who don't know, IaRCS is a low distortion projection that fits several counties quite well, and appears to have been defined at the initiative of the DOT. As soon as everyone's software implements it (if not already done) it seems like the best approach. I made a spreadsheet to convert from lat-lon to IaRCS and was working in that system until I noticed that SPC fit the plat better.
What I have now is a scaled and rotated version of SPC, using the angles deduced from the plat rather than bearings, and using a single local datum adjustment factor to convert to IaRCS. I'm sure that is plenty good for search coordinates, especially since both are Lambert projections.
But I'm interested in the theory, too. I'm wondering if I should take the trouble to go through a long conversion process. I'm guessing the measurements behind the plat were made at least partly with GNSS. Would this sequence theoretically give me a more accurate estimate of the positions measured for the platted survey, due to avoiding the approximation of a single scale factor?
1. Convert my lat-lon for one plat corner to SPC.
2. Temporarily hold that point (close enough to get scale and mapping angle) and convert plat bearings and distances into SPC coordinates for other corners
3. Convert SPC coordinates of all corners into lat-lon
4. Convert lat-lon into temporary IaRCS coordinates
5. Convert temporary IaRCS coordinates to angles (or bearings?) and distances
6. Least squares fit my two OPUS points with those angles and distances to get best IaRCS coordinates
I think this would give me a slightly different, and theoretically better, estimate of the positions. It won't be more than tenths different, if that. But is it really a better estimate in theory than just scaling and rotating?
Don't you just have a metal detector and shovel?
Scott Zelenak, post: 422443, member: 327 wrote: Don't you just have a metal detector and shovel?
I do. But those aren't very efficient if you don't have a good idea where to look in a stretch of woods adjacent to a RR ROW with its left over hardware and other accumulated trash.
Bill93, post: 422417, member: 87 wrote: But I'm interested in the theory, too.
