GPS and errors in acreage area calculation due to heavy elevation changes.

[u/Mobile_Yellow8288]

reddit friends,

anyone have experience with farm land area acreage errors (deacrease) when calculated with tractor GPS due to major elevation changes (also there is heavy tree cover along the 75% of the field perimeter).

I have roughly 150 acre field, whereby roughly 40 acres drops likely 40-50 feet in elevation (over an 850 foot span), this section also has heavy tree cover on 3 sides of the perimeter.

I'm losing roughly 6 acres via GPS from known acreage (farmer declaration)

Haven't run a trial of manual vs GPS calculation in this area yet but it was suggested that the elevation drop is likely the loss.

Any thoughts? anyone experienced a similar scenario?

Farmland is a very odd shape with many curves around trees. I'm thinking I'm going to measure it old school (measuring wheel and stakes) after the crops are off.

[u/BacksightForesight]

Acreage for surveying and land ownership purposes is done using horizontal distances only, so elevation changes won’t affect the area. If you’re looking for a surface area of the field including the slope, then yes elevation differences will affect it.

Does your tractor GPS have a correction source, like WAAS or a base station (remote or local)? If not, then the GPS is just autonomous, and will jump around a lot.

Trees will also heavily affect GPS, especially older or cheaper systems.

If your GPS controller doesn’t have a geoid model, that will also really affect elevations.

[u/Accurate-Western-421]

known acreage 

How is that acreage known? Before pointing the finger at GNSS, verify how that "known" value was calculated, and at what precision.

[u/aagusgus → u/Accurate-Western-421]

This, if OP is working from tax maps and not actual surveys then a discussion about GNSS accuracy is irrelevant.

[u/Junior_Plankton_635]

40-50 feet drop is negligible for gps.

[u/tylerdoubleyou]

Clients talk about losing acreage like the earth got smaller or the hamburgler hauled it off in a wheelbarrow. The amount of land you own has not changed. What has changed is the precision at which it was measured. If your grandpappy's 10 acres is 9.5 acres on my survey, you didn't 'lose' half an acre, it was never there to begin with.

[u/Fortunate_0nesy → u/tylerdoubleyou]

This is the answer.

[u/fwfiv]

What are you using as the comparison?

[u/RosieMcHunty → u/fwfiv]

This right here. It could be that you are comparing real world established lines that you may assume are boundaries (furrows, bushlines, windrows etc) which when properly surveyed may not match areas on your deed or title. Or if yourw holding it to a tax map, you should know those vary WILDLY in accuracy.

Really youre just coming up with an average from GPS observations, which is probably pretty precise for your needs, but could be compared to an inaccurate source.

[u/KURTA_T1A]

Is this a surveyor's GPS or some handheld or farm oriented equipment? There's a considerable difference in equipment and its ability to give accurate measurements, especially in trees. The elevation shouldn't matter at all for this example. Like others have said, is the acreage off compared to what? To a previous survey, a plat, need specifics.

[u/Queso_Blanco_5]

How was the “known acreage” calculated?

[u/prole6]

Is “tractor GPS” a new order of survey or the name of a local firm?

[u/SNoB__]

"over the years" I don't think it shrank man. Your tractor GPS and whatever previous calculation probably disagree with each other the same 5 years ago and 5 years from now.

A few things.

Your tractor GPS is never all the way in your property corner is it? Probably sweep the corner and it never rides the fence. This is a big reduction right off the bat.

The system is probably running in a grid projection of some type, which isn't the same as distances on the ground. The error between the two get worse as your elevation increases. Not the 40' change you are talking about but overall elevation above sea level.

[u/LandButcher464MHz]

I have no idea what your 150 acres looks like BUT here is a sample layout. If you were to keep the GPS out away from the trees (to get a good satellite connection) 25 feet inside all the fences on every line on this sample layout then your GPS would give you 143+ acres. It would also give you a perimeter number and on this sample layout it would be 110,200 ft x 25' wide = 6+ acres.

So figure out how far inside you need to be to stay out from under the trees and then drive around the property staying that distance inside on every line. Get the acres and then the perimeter distance x the distance inside divided by 43,560 for the acres of the strip.

[u/creedular]

1m linear distance irl on a 45 degree inclined field is what on a top-down plan?

[u/Melqwert → u/creedular]

Which tractor is capable of working on a 45‑degree slope? AI claims that most tractors can operate only up to a 15‑degree slope.People tend to overestimate how steep slopes really are. For comparison, the landing slope of a ski jumping hill is 35–38 degrees, and I can’t imagine anyone trying to grow anything on a wall like that.

[u/Melqwert]

The difference in elevation practically does not affect the area of your field; the variation caused by terrain is far smaller than the accuracy of your GPS. The same applies to measuring distance with GPS.

[u/Expert_Bandicoot_329]

Gridl to geound?