GPS for Measuring Straight-Line Distances?


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Drakejake
July 12, 2007, 01:34 PM
This may be the wrong place for this question, but I bet someone here knows about this. I want to measure straight-line distances on a tract of wooded land. Can I do this with one of the GPS handheld devices made by Garmin, for example? I am talking about distances of up to a half mile in hilly terrain.

Thanks,

Drakejake

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igpoobah
July 12, 2007, 01:40 PM
Yes, but accept the fact that you could be several feet off just by the nature of how GPS works...

If you have a GPS that can do averages, you can get it closer...

GRIZ22
July 12, 2007, 02:12 PM
Keep in mind GPS measures distances from above on a flat plane (works fine on the ocean) but will not take into account hills. Not a problem if you were measuring for a range. The major factor is what igpoobah mentions.

ArmedBear
July 12, 2007, 02:15 PM
Measure them?

Not directly. The odometer feature tracks actual distance walked or driven, so that would include up and down hills.

What you can do, though, is put in a waypoint at each corner, and calculate the distance between the points.

Note that waypoints include altitude by default, but some GPS's will allow you to remove it, depending on how you want to do your "straight-line" calculation. Do you want the line to include slope in the calculated distance, or to assume that both points are at the same altitude and calculate a straight line that way? My guess is that it's the latter.

A WAAS-enabled GPS with location averaging can be pretty close, within a few feet. My eTrex Vista CX has the features mentioned, and mapping, so you can draw your plot in a topo map, print it out, and add locations for landmarks, structures, ponds, etc. It's a fun toy and a useful tool, though I started out with the most basic model and got familiar with that first. The Vista has a lot more features to wade through.:)

Drakejake
July 12, 2007, 03:18 PM
Yes, ArmedBear, I want distance in a flat plane above the earth because I am trying to establish property boundaries. I assume that if the plat shows a boundary to be a quarter of a mile long, this means that the distance is a straight line on a plane above the hills. So walking the hills would give you a false distance, right? An acre, for example, consists of a flat plane above the surface of the earth, not distances following the surface. Consider a crevass which is 100 feet wide and 500 feet deep. The land included would be 100 feet wide, not 1000 feet wide, if this makes sense to anyone.

Thanks,

Drakejake

ArmedBear
July 12, 2007, 03:47 PM
Ah.

There's something BETTER you can do.

Get something like the Vista Cx and Garmin's Topo software (another $100). Mark the waypoint in the field, with averaging, WAAS, etc. to maximize accuracy.

Then, upload the waypoint to your computer, draw the property boundary on the computer map starting with the known marked waypoint. Create waypoints for the corners. Download these new waypoints to your GPS and download the topo maps to the GPS as well.

Go back into the field with your GPS, now with a topo map in full color on the screen, and boundaries marked, find the points you downloaded, and mark them with flagging tape, stakes, etc.

If you do it just right, you'll be really close, and the GPS with topo map will help you find your way through the hills.

GPS's are neat!:) They do take some commitment to learning the bells and whistles. I recommend a book, too, so you understand how the thing works, especially WRT different types of topo maps, compass declinations, etc.

I have the Stephen Hinch book, and can recommend it. Well worth the $11, to get the most from your $300 GPS and $100 mapping software.:)

http://www.amazon.com/s/ref=nb_ss_gw/104-6013799-6277530?initialSearch=1&url=search-alias%3Dstripbooks&field-keywords=GPS&Go.x=17&Go.y=10&Go=Go

Alphazulu6
July 12, 2007, 04:04 PM
My Garmin GPS 60cs and Garmin Vista both do straight line +-3 feet generally. You can also do area square footage calculations on both of those and any of the better ones offered by other brands. And to beat the dead horse.... but it will be a tad off unless you do it a few times and take the running average (measure/walk it four times, add them all up, divide by 4).

Good Luck!

ArmedBear
July 12, 2007, 04:07 PM
The Vista Cx will do the averaging for you, though you have to read up on how to do it just right.

Doesn't the Vista?

Drakejake
July 12, 2007, 06:30 PM
ArmedBear and other commentators,

I have the plat with metes and bounds (distances, angles, and set curves). I do not see a fixed point on this map, i.e., a pin or physical marker or lat/long. The southern boundary is a road and so I know where that boundary is. I would like to mark a GPS point on that road and then walk up the opposite hill and know the straight-line distance between the two points. I am considering the Garmin CSX (something like that) but am not sure whether I can use it to measure between points, rather than a walked or driven route. The land is very hilly and thickly wooded. I have online topo maps and online county tax maps with the drawn boundaries. What I do not have is some way to connect something on the maps to something on the land itself. Sorry, my only training in this was as a Boy Scout many years ago.

By this way, there is some connection between my question and guns, because this is land I just purchased for target shooting and camping.

Thanks,

Drakejake

Alphazulu6
July 12, 2007, 06:39 PM
The Vista Cx will do the averaging for you, though you have to read up on how to do it just right.

Doesn't the Vista?


*scratches head* I havent got to that chapter yet in the three years I have owned a vista. I know it can do alot. I have never actually tested the Jumpmaster T-10 Delta parachute altimeter countdown either (I dont think I would want to be lookin at my vista while taking the plunge anyway) hehe.

They are remarkable hand held pocket carry devices :D

ArmedBear
July 12, 2007, 06:43 PM
whether I can use it to measure between points, rather than a walked or driven route

Yes.

If you can find just one point and set that as a waypoint, you can project X distance in Y direction from that point and create a new point there. Then you can walk towards it and you'll be able to see where you are WRT the new point.

What I don't know anything about is surveying, or the sorts of numbers surveyors will give you.

Drakejake
July 12, 2007, 07:40 PM
Thanks, guys. I used Pricegrabber to find the best price and ordered the Garmin CSX from PCNation.

Drakejake

kungfuhippie
July 12, 2007, 07:52 PM
Okay, this is my job!!!!
Surveyors use GPS to get very accurate measurements 1/100" The trick is to leave the GPS running at a point for a long period (12hrs+) this will allow it to read off of several satellites and get a more precise point. The Plat map should have a benchmark of some sort listed on it. Usually a nail, brass dish, steel or lead pipe. Everything is referenced off of this benchmark seeing as how back 100+ years ago when the USGS surveyed (or earlier than that in TN) Because it was very easy to base everything off of fixed physical points wrather than latitude and longitude. If you are doing this for curiosity, great. If you plan to use it for putting in a fence or determining what is yours, spend a few hundred and have a licensed surveyor mark you property. Then it's legally defined. Many surveyors will do this type of work and depending on the number of points (4 corners or a zig-zag property) will do it fairly cheap. If you can find a buddy who knows how to use a transverse or total station you could bribe him with some adult beverages or ammo etc. to help you out too.

Drakejake
July 12, 2007, 09:58 PM
This tract is 38 acres and I believe a stake survey would cost thousands. However, one boundary is a paved road and the other two boundaries are straight lines, about a half mile and a quarter mile, respectively. The tract is part of a subdivision and the plat I have is one of about eight total. I do not see any reference to a physical marker on my map.

Drakejake

22-rimfire
July 12, 2007, 10:58 PM
You may be able to use an adjacent parcel to find your approximate corners. I have done this and usually there is something to base your field measurements off of. Surveyors will often do this if your property has no corners marked. But to do it right or even approximate, you should probably hire a surveyor.

Since there is a platt map, the corners may still be marked. I would spend a fair amount of time looking for the pins set for at least one of the corners. Remember, the corner along the road will not be at the edge of the road by off the road 10 feet of so. Road should have the rightaway width wmarked hich you can measure from the center line of the road. One of those deals that have the big magnet (or metal detector) will help you find the pins if they are sunk to ground level.

KD5NRH
July 12, 2007, 11:24 PM
This tract is 38 acres and I believe a stake survey would cost thousands.

Shouldn't be that bad; we just had a 10ac rectangle done and it was under $500.

Two points and one boundary were defined, and two others partially defined, which likely helped a lot with the price, but the remaining points required some thrashing around in briars to get to, which likely didn't do good things for the price.

What we did, at the surveyor's suggestion, was to get our estimates with the GPS, and try to clear paths for him so he wouldn't have to spend the extra time with a machete.

Needless to say, we missed by about 12ft on the endpoints, leaving the line about 6ft into the brush, more due to my calculations being a bit off (neglected the shift in magnetic declination since the previous survey, among other things) than the GPS being that far off.

00 buck does wonders for trimming out problematic tree branches, BTW.

kungfuhippie
July 12, 2007, 11:55 PM
Well if you are just interested in 1/8 of the 38 acre tract (4.75 acres) you could probably have gotten those lines two surveyed for less than the GPS, though the GPS is always fun to have (and useful). Going cheap on land is like going cheap on pistols-you can really lose big for it.

Bwana John
July 13, 2007, 01:36 AM
Simple trig. :)

Put GPS in UTM mode.
Take GPS to both location.

Call one location (1), the other (2).
Take UTM quardinates of both locations, you will get a 2 numbers at each location, N-XXXXXXX & E-XXXXXX

Remenber A^2+B^2=C^2?

A=distance between N(1) and N(2)
B=distance between E(1) and E(2)
C=distance between the 2 points

The final math looks like;
(N1-N2)^2 + (E1-E2)^2 = (distance between the 2 points)^2

(if you are not familiar with UTM, the distance will be in meters, depending on your GPS unit the error should be <3 meters)

kungfuhippie
July 13, 2007, 02:07 AM
Bwana John.
That won't be any more accurate. Let the GPS collect info from several satellites (the $50k+ units surveyors use site for 24 hours when possible before they are considered on point) IF the unit has the batteries for it give it at least a good hour, the longer you wait up to 24 hours the more precise the reading will be. A handheld unit will never be dead on, it's like a Walmart scope: good up to it's designed limits. But getting a boundary survey, minimum should be included in the cost of buying large unmarked property. Last thing you want is to put in a fence 6" onto your neighbor's land and get sued over it. Even if you win it's expensive to fix. In my expirence lawyers cost more than licensed surveyors...And a surveyor is more fun to shoot the breeze with.

308win
July 13, 2007, 07:52 AM
I have permission to shoot on a fairly hilly but open parcel. I have often thought of using a GPS to calculate distances by taking the lat./long measures at different points. Why wouldn't this work? If I am off by a couple of yards I don't mind as the difference in bullet drop will be negligible.

budney
July 13, 2007, 08:04 AM
Yes, ArmedBear, I want distance in a flat plane above the earth because I am trying to establish property boundaries.

For general purposes, like knowing where you can hunt, the GPS is plenty adequate. If you're planning to build a fence and post it "no trespassing," then you really should get it surveyed.

--Len.

MaterDei
July 13, 2007, 08:25 AM
A Roundtable discussion, cool. I am always so impressed with the breadth of knowledge found here on THR.

Back in my artillery days we would survey in firing points. We used remote theodolytes then. Does anybody still use RTs? :)

Bwana John
July 13, 2007, 10:38 AM
I have often thought of using a GPS to calculate distances by taking the lat./long measures at different points. Why wouldn't this work
You "can" do in in Lat/Long but unless you are measuring exactly N/S or are on a great circle the math is much, much, harder.
Learn how to use UTM.
Bwana John.That won't be any more accurate
depending on your GPS unit the error should be <3 meters)I agree he should hire a LS for property boundrys, as stated the percision depends on the GPS unit.

kungfuhippie
July 13, 2007, 11:34 AM
Does anybody still use RT
Yes, we just use different names for them.

I guess what we really want to know is why he needs to know the dimensions of his land? For kicks? for hunting? To build? To prove that the neighbor built a swimming pool half on his property? (not joke, saw this once)

Drakejake
July 13, 2007, 11:50 AM
Here in Nashville, a staked survey of a one-third acre lot can cost $1,000. Generally, the bigger the tract, the more the cost. All I really need is to have three points marked. But I will call some surveyors working in that county and get some idea of costs.

Why do I want to measure this land? I want to know where the boundaries are. For example, there is a hill on the property and I would like to know how far up the hill is the boundary. Is there enough land for a home site? When I go up the creek, I would like to know where my land ends. My neighbor has no trespassing signs up. I would like to know if they are in the right place. At any rate, I think a GPS would be fun and useful to have.

Thanks,

Drakejake

308win
July 13, 2007, 12:04 PM
You "can" do in in Lat/Long but unless you are measuring exactly N/S or are on a great circle the math is much, much, harder.
Learn how to use UTM.

Don't GPS calculate distance between two waypoints (is that the correct term) for you? What is UTM? I don't have a GPS but have considered getting one.

kungfuhippie
July 13, 2007, 01:34 PM
straight distance between two points using latitude and longitude...just a mouse click away.
google earth-it's free. put in two "pins" at the correct lat/long and then use the measure tool to find the distance in feet, yards, meters, miles, km...

$1000 is about right for it. But to get anything beyond an answer to curiosity-you'll need a professional. For example if you find that you're neighbor's signs are 10' on your property with your GPS you won't be able to doing anything legal about it, you'll then need to hire a surveyor, and a lawyer... Your boundary must be tied to something. IT is probably tied to the tract map-which is public record so you can get a copy of it along with the legal description, this will help you establish a starting point that it's all based on as well as identify where some markers might be to help. If the tract map was done recently (within the last 25 years) you should have corners already marked, might need a metal detector to find them though. (not a magnet since most often corners are marked with brass or lead or other non-ferrous materials)

fletcher
July 13, 2007, 01:48 PM
To get straight-line distances:
- Mark a waypoint (let's say Waypoint 1)
- Walk to second location (Waypoint 2)
- While standing at Waypoint 2, select Waypoint 1 as if you were going to use the GPS to navigate to it
- Keep still, and it will show you the straight-line distance from Waypoint 2 (since you're standing there) to Waypoint 1

Or, import the waypoints to something like Google Earth and measure the distance.

Drakejake
July 13, 2007, 02:07 PM
I have used Google Earth but the aerial map for my land is green and out of focus. Yahoo, for example, has my land with a clear aerial view.

Drakejake

Bwana John
July 13, 2007, 02:42 PM
What is UTM?
http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system
"google earth" , "While standing at Waypoint 2, select Waypoint 1 as if you were going to use the GPS to navigate to it"

You guys are taking the "fun" out of it;)

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