Using GPS to lay out a field

[quicksandfarmer]

I have a good idea where my corners are. My 20 acre, nearly square parcel is bounded on two sides by two intersecting roads, so the exact corner there is unimportant... the practical boundaries on those two sides are the two roads. One boundary is a state wildlife area, and it's roughly marked with paint on a line of trees. The two corners that interest me have been surveyed. One is supposed to have a pin, but I was unable to find it. The other is a "stone", and sure enough, there's a painted stone along the property line I share with the state, right where my GPS said it should be, within the error of the GPS.
There's a property marker across the road from the corner with the missing pin, but it's not got a surveyor's cap on it, it's a painted "T" post, but I believe it's pretty close based on my GPS and measuring from corner at the intersection of the roads.
I think, but am not certain, that my boundaries are recorded with lat and long coordinates. I'll have to dig out the document to be sure. Three of the corners are on roads, so I have a pretty good view straight up, but there are trees on both sides. That fourth (the painted stone) corner is in the woods, but my handheld GPS has reception there.
Thank you all for the input! It's much appreciated. I may just buy that kit to have it, I'm a ham radio operator, fly model airplanes, and bought a hand-held GPS WAY back when they first became generally available and were WAY too expensive. In other words, I like my gadgets.

If you have latitude and longitude PM me and I will draw a 1-yard circle on the aerial photo from Google Maps at that exact spot and send it to you. If there are landmarks on the ground the aerial photo can get you very close to the exact spot.

 

[dodge man]

It would be really rare to have a lat and long on a boundary survey but maybe it could be a regional thing.

 

[s219]

It would be really rare to have a lat and long on a boundary survey but maybe it could be a regional thing.

I have never seen it here, but a lot of property owners get really confused thinking their property line bearings (in deg-min-sec) are a position coordinate. Can't tell you how many times someone has asked me for help with that .

 

[quicksandfarmer]

So it turns out the precision is closer to 2 cm than 1 cm. The DGPS unit is capable of 1 cm precision, but its output is in NMEA format. The NMEA format has latitude and longitude in degrees and minutes, the minutes are reported with five decimal places. So the minimum precision is 0.00001 minutes. At my location one minute of latitude is about 1800 meters and one minute of longitude is about 1450 meters. So 0.00001 minutes is about 1.8 cm of latitude and 1.45 cm of longitude. For my application so far that's fine. U-blox does have a proprietary format that gives greater precision if needed.

 

[dodge man]

As a rule surveyors do not work in latitude and longitudes although it is a mathematical conversion to state plane coordinates from lat and long.

One thing, speaking as a surveyor, that would limit the use of your system is the interface. We use data collectors that give a lot of information. QA/QC numbers being important. When using ours one thing I keep an eye on the display is if the solution is fixed, float or autonomous. Fixed means cm level accuracy, float means something is wrong, such as to many trees, and autonomous means your radio link is down.

A couple of things, GPS does not work in tree cover. There have been huge improvements in how it works in the trees but once the leaves are out, you just can not get a fixed solution.

Also the precision of GPS is not that great. If you take a single measurement on a point, shoot it the next day, it won稚 check that great. I am talking an error of a half inch or more sometimes. What this means is for critical measurements such as bridges, building layout and paving operations, it is not good enough. Say you use GPS to shoot property corners 20 feet apart, you would get a better measurement with a 30 foot carpenters tape measure. Where it shines is longer distances, places you do not have line of sight, and where precise measurements are not critical.

Another point is GPS does nothing to improve boundary surveying. It makes it easier, it is another tool but the answer is not floating around in space in satellites. There is a list of calls in order of importance, such as call to adjoined, natural monument, man made monument, bearing, distance, area and coordinates. Coordinates is last on the list meaning you give it the least weight. That is because you do not always know how the surveyor got the coordinates. What did he start from, what equipment did he use? To many variables now to move it to the top of the list.

 

[pmsmechanic]

Question here. So do you need a pin finder to find pins or will a metal detector or similar work? I'd like to find the pins on my property without spending a lot of money. I know where they are within a few feet but I would like to know exactly.

 

[BrokenTrack]

Question here. So do you need a pin finder to find pins or will a metal detector or similar work? I'd like to find the pins on my property without spending a lot of money. I know where they are within a few feet but I would like to know exactly.

I use a metal detector. It does more than just find the pins. A lot of times it might pick up on old barb wire, page wire fencing, or hardware from an old fence long since rotted. All these may indicate where the property line is.

Another indicator here is big trees. Many times the biggest trees are on the property line because neither landowner dared to cut the trees down. Along the same lines, a change in tree species, can also be an indicator. The way it works here is: on new growth, softwood comes up first, then after 80 years or so, it starts switching to hardwood. So if you are near the line, and suddenly there is a change from pure hardwood, to softwood, you can bet that is where the line is. I would never lay out a property line using that alone, but it will let me narrow in on where to take my metal detector and start looking for barb wire, page wire fencing, and line pins.

Here in Maine, only surveyors can set a pin, but anyone can re-do what has already been laid out for a line. What I am finding lately though is, most landowners have no clue where the property lines are. A few weeks ago I had a neighbor sell my land on me, but granted it is very remote here.

 

[dodge man]

A metal detector will work. The ones we use tend to only find iron, not coins and other stuff the treasure hunters look for. I probably dig up 3 pieces of junk for every corner I find.

 

[quicksandfarmer]

As a rule surveyors do not work in latitude and longitudes although it is a mathematical conversion to state plane coordinates from lat and long.

One thing, speaking as a surveyor, that would limit the use of your system is the interface. We use data collectors that give a lot of information. QA/QC numbers being important. When using ours one thing I keep an eye on the display is if the solution is fixed, float or autonomous. Fixed means cm level accuracy, float means something is wrong, such as to many trees, and autonomous means your radio link is down.

A couple of things, GPS does not work in tree cover. There have been huge improvements in how it works in the trees but once the leaves are out, you just can not get a fixed solution.

Also the precision of GPS is not that great. If you take a single measurement on a point, shoot it the next day, it won稚 check that great. I am talking an error of a half inch or more sometimes. What this means is for critical measurements such as bridges, building layout and paving operations, it is not good enough. Say you use GPS to shoot property corners 20 feet apart, you would get a better measurement with a 30 foot carpenters tape measure. Where it shines is longer distances, places you do not have line of sight, and where precise measurements are not critical.

Another point is GPS does nothing to improve boundary surveying. It makes it easier, it is another tool but the answer is not floating around in space in satellites. There is a list of calls in order of importance, such as call to adjoined, natural monument, man made monument, bearing, distance, area and coordinates. Coordinates is last on the list meaning you give it the least weight. That is because you do not always know how the surveyor got the coordinates. What did he start from, what equipment did he use? To many variables now to move it to the top of the list.

Thanks Dave. I wouldn't dream of using this for boundary surveying or anything where anything real is at stake. That said, I have a 300' tape measure that I use all the time, the idea of being able to draw a line on a map on my computer and then being able to go out and walk that line just seems like it could replace a lot of the things I use that 300' tape for. I could see using it for fencing, planting trees, plowing rows, etc. I doubt I'm getting 1" accuracy with the tape anyway. I built a 96' greenhouse with 25 hoops and I would have loved to get 1" accuracy placing those hoops.

What got me started on this thread was an earlier thread about using laser sensors to set the height of a box blade. There's a lot of possibilities for automated control once you can determine your location relatively accurately. I haven't even thought about what you could do behind the wheel of a tractor. The ultimate project would be some sort of autonomous vehicle, maybe something that mows the fields while I do something else. But that's advanced stuff.

The U-Blox unit will tell me if the solution is fixed, float or autonomous. (I had forgotten the terminology in an earlier post). I change my display base on what it tells me. If I'm getting fixed the current location is marked with the small blue X:

If I'm getting a float solution I mark it with a larger, yellow X:

The idea is the size of the X roughly represents the accuracy. If I'm getting autonomous it's a much bigger red X.

Agree that you can't always get a fix, I spend a lot of time at some sites trying to get that blue X. You need line of sight to the satellites. Generally fields work pretty well.

I appreciate your comments, it is very helpful to hear from someone who knows a lot more than I do about the real-world use of this technology.

 

[BrokenTrack]

I have noticed over the last 40 years that the USDA has improved their accuracy. For years they said we only had so many acres, which was 20 acres less of farm land that we always said we had. But I always assumed it was because they go by a flat area, where as we live in very rolling hills. It would be like taking an 8x 11 sheet of paper, and tracing around it. Then taking that paper and twisting it and looping it...then tracing around it. The same square area would fit into a much smaller space. But now I noticed, their acreage amounts have increased to where it matches what we always said it did, I assume because with GPS they can take into account land contour better.

It has gotten to the point of accuracy now, that with GPS and LIDAR, I can go on the USDA website, Web Soil Survey, and map out my fences. I can take the footage and calculate my wire, my fence posts needed, and everything, and know I will only be off by a few feet in actual construction. That is pretty neat since I can do all this on a rainy day, and never step outside.

It is the same with soil types. My soil samples physically gathered, and sent in, always have matched what Web Soil Survey says it is.