Ground Distance, Grid Distance, and a Developer Who Wants 20 Acres
You are preparing a boundary survey of a 20.000-acre tract in Gwinnett County between two original 1-inch pipe corners that you have found and verified. The total-station distance between the pipes, with instrument and atmospheric corrections applied, is 2,106.42 ft; the 1968 deed calls the line as 2,106.00 ft 'more or less.' The tract lies at an average ground elevation of about 1,080 ft and roughly 95,000 ft west of the central meridian of the Georgia State Plane zone; from the published projection you compute a combined factor of 0.999620. Area computed by the coordinate method from grid coordinates is 19.9846 acres. The developer buying the tract wants the plat to 'show the 20 acres the deed says' and wants distances labeled as measured on the ground, with no scale-factor note, because 'the note just confuses the title company.'
1. Compute the grid distance between the two pipes and determine whether the deed distance is inconsistent with your measurement. Show the computation and explain what may and may not be compared.
Read the worked answer
Grid distance = ground distance × combined factor:
2,106.42 × 0.999620 = 2,105.62 ft
The meaningful comparison is measured ground to record: 2,106.42 − 2,106.00 = 0.42 ft, or about 1:5,015. For a 1968 deed qualified 'more or less,' that is ordinary agreement and confirms the pipes.
The comparison you must not make is grid distance to deed distance. 2,105.62 against 2,106.00 looks like a 0.38-ft error, but it is not an error at all: the deed distance is a ground measurement, while the coordinate-derived number lives on the projection surface about 1,080 ft below and 95,000 ft off the central meridian. Treating the 0.38 ft as a discrepancy invites a 'correction' that would move a sound boundary to make a projection artifact disappear.
State the basis clearly: the boundary was measured on the ground; coordinates and areas computed from them are grid values; the two differ by a known, documented factor, not by blunder.
2. The developer demands that the plat show 20.00 acres. Evaluate the 19.9846-acre grid area and state what area you will publish.
Read the worked answer
Projection shrinks area by the square of the scale factor:
CF² = 0.999620² = 0.999240
Ground area = grid area ÷ CF² = 19.9846 ÷ 0.999240 = 19.9998 acres, which rounds to 20.00 acres.
So the observed 0.0154-acre (about 670 ft²) shortfall is entirely the projection, not a boundary problem. That is a useful finding for the client, but it does not license you to disregard the projection or to back-figure a number.
Publish the area you actually computed, to the precision your survey supports, and identify the basis. If you show ground area, say so; if you show grid area, say so. A plat that quietly prints '20.00 AC' with no basis note is a claim the record does not support, and a later surveyor who recomputes 19.98 from your coordinates will have no way to tell whether the difference is projection or blunder. The correct answer to the developer is: the ground area is 20.00 acres, the grid area is 19.98 acres, and the plat will say which is which.
3. Draft the basis-of-bearings and distance note you would place on the plat, and explain why each element is there.
Read the worked answer
Suggested note:
'Basis of bearings: Georgia State Plane Coordinate System, East Zone, NAD83 (2011), grid north. Distances shown are ground distances at the mean elevation of the tract and are not grid distances; multiply by combined factor 0.999620 to obtain grid distances. Area shown is ground area; the corresponding grid area is 19.98 acres.'
Why each element:
- Basis of bearings fixes the orientation so any later surveyor can rotate your plat onto their own control instead of guessing which north you used.
- Ground versus grid tells the reader that the numbers are not directly comparable to state plane coordinates, and gives the exact factor to convert.
- Area basis prevents a false discrepancy from being read as a boundary error.
This note is also what an ALTA/NSPS-style product expects, and it protects you if the title company, lender or a later surveyor recomputes the geometry. The developer's objection is a communication problem, not a technical one: put the numbers in a cover letter in plain language, but keep the note on the plat.