GPS-Derived Coordinates Versus the Record Plat in a Subdivision Retracement
You are retracing Lot 7, Block 2 of a 1962 subdivision plat recorded in the local recording district. The plat gives lot lines by bearing and distance from monuments set at each lot corner, with a stated basis of bearings tied to the section line. The plat does not state a datum, but the ties to the section corners are consistent with a 1962 state-plane or local system. Your RTK GPS (NAD83, epoch 2010) gives coordinates for the two found original lot corners that differ from the plat bearings by about 25 seconds of arc, and the measured distance between them is 0.18 ft longer than the plat distance. A 1998 survey of the adjoining lot, recorded, used a different datum and shows the common line 0.3 ft from where your GPS places it. The lot is 62.5 ft wide. The client wants you to 'just use GPS' because it is more accurate.
1. Which evidence controls the location of the lot lines, and how do you handle the 25-second and 0.18-ft discrepancies?
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The found original monuments at the lot corners control the location of the lot lines. The plat bearings and distances are evidence of where the original surveyor intended the lines to run, but the monuments are the physical embodiment of that intent. Your GPS-derived positions are a measurement of the monuments, not a replacement for them. The 25-second bearing difference and 0.18-ft distance difference are within expected measurement and monument-setting tolerance for a 1962 survey and do not justify moving the corners; they are evidence of the original survey's accuracy, not of a different boundary. You hold the found monuments, use the plat bearings and distances as a check, and report the discrepancies as part of the survey record. GPS accuracy does not override monument control; it is a tool for measuring and for finding evidence, not a source of boundary location.
2. The 1998 recorded survey places the common line 0.3 ft away. What is the effect of that recorded survey, and what should you do about the discrepancy?
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A recorded survey is evidence of the 1998 surveyor's opinion and of the monuments and measurements then found; it is not itself a conveyance and does not move the boundary. If the 1998 survey was based on different or inferior evidence (for example, it used GPS coordinates without holding original monuments, or it used a datum transformation incorrectly), its line is in error and does not control. If the 1998 survey held original monuments that are now gone, it may be valuable secondary evidence of those monuments. You should: (1) search for and evaluate the monuments the 1998 survey relied on; (2) compare its basis of bearings and datum to the record plat; (3) document why your line differs; and (4) if the discrepancy affects the adjoining owner, consider notifying them and, where appropriate, filing a record of survey or a boundary line agreement. You should not simply adopt the 1998 line to avoid conflict, nor ignore it.
3. How do you handle the datum and epoch issue in your own survey, and what must appear on your plat or report?
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You must state the datum, the realization/epoch, and the projection or coordinate system used, and you must show how your coordinates relate to the record basis of bearings. Because the plat has no stated datum, you establish the relationship by holding the original monuments and the record bearings, not by forcing the plat onto NAD83. You transform or rotate your GPS-derived coordinates to the record basis of bearings, document the transformation parameters and residuals, and report the coordinates in a clearly identified system. On the plat or report, show: the datum and epoch; the coordinate system and units; the basis of bearings and how it was derived; the found monuments and their descriptions; the record versus measured comparisons; and a note explaining any transformation. Never present GPS coordinates as if they were the record boundary without this documentation.