Joined: 05 Jun 2008 Posts: 12415 Location: New Jersey
Sat Apr 16, 2011 2:08 am
Depends on what you expect. I have spent a lot of time with this on my own land. In fact, have just started playing with this further last weekend in an effort to be more accurate. Using my 60csx, I used waypoint averaging of about 3000 samples each and the unit was estimating 3.9 foot accuracy the first time and 4.3 feet the second. The two points I recorded (at the same location) are about 7 feet apart.
Using the Oregon to average waypoints over the same period, it claimed 100% confidence in the locations. The two waypoints I recorded varied by 10 feet. All the points recorded by both units fall into a circle with a 20 foot diameter.
This is really about as good as it gets with any consumer GPS - the eTrex should be very similar. I was using WAAS for these, which improves accuracy, and the leaves have not yet appeared on the trees here.
Consumer model GPS'es are spec'ed at +/- 5 meters. This means that if you record multiple points at the same known location, they should all fit within a circle with a 10 meter (33 foot) diameter. The results I described above are within that margin of error - a little better actually.
Now about your survey, are you sure it contains coordinates? In my experience, that would be unusual (unless you requested it). Usually the survey shows a starting reference point, then gives vectors describing how to get to the next stake. So, in your example, it might mean to go 500 feet (or whatever) on a heading of 270 degrees 3 minutes to reach the next point.
You can do this on Garmin handhelds using something called waypoint projection. However, the results will not be very accurate because of the number of decimal places used in the calculation.
Bottom line is that it can be fun to play with, and can help you understand your property, but it's nothing like a "survey". As I mentioned above, if you visit the exact same place on different days, the GPS could record points that are 33 feet apart and it would be functioning up to specs.
In any case, I certainly wouldn't call an eTrex "ideal". Also note that there is a big difference in features between various models in the eTrex line. I have found the use of GIS software along with property parcel data that can be downloaded from my county and high resolution aerial imagery have given me a better understanding of my own land.
I've used all this to make my own map for the GPS to use in the field. But this all gets pretty complicated.
I don't quite understand how to read my plat map, but the boundaries of my land do include a legend with various icons for rebars, stakes, posts. The corner of my property has a "1/2 set rebar" with 270.00'03" 305.25'. I'm assuming that the "305.25' is the distance to the next point? I guess I'll upload a part of it if it helps...
Joined: 05 Jun 2008 Posts: 12415 Location: New Jersey
Sat Apr 16, 2011 11:45 am
Yes. that is exactly what I thought. Those dots are some kind of survey stakes (such as rebar). Starting from the bottom (I assume - can't tell from such a small scan) you would go 305.25 feet on a heading of 270 degrees 3 minutes to the next stake. Then 1056.28 feet on a heading of 270 degrees to the next, and finally 142.78 feet on the same heading to the stake at the top of the page.
Can you explain what you would like the GPS to do for you? Are you trying to plot this on a map? Or are you trying to find where the stakes are in the woods?
There is no "coordinate" information given in the example you posted, so no way to locate it on a map. You could go to the stakes and take GPS readings, but they will be subject to the kind of errors I mentioned above. With waypoint averaging, you leave the GPS in one spot for awhile (like an hour) and it takes one reading each second, then averages them all. This can get you pretty close, if the conditions are ideal. But probably no closer than 10 feet, which is what I have seen in my own experiments.
My own land is densely wooded and bordered by a state forest. I have only found a stake in one corner. I don't think there have even been stakes in the other corners because they are very overgrown and inaccessible. I used my gps'es to roughly plot the property lines so I could post them. This is been an ongoing project over several years, and I've repeated it enough that I'm pretty confident where the lines are, but since it's just state forest on the other side, nobody is going to really care if I am off by 10 feet.
Many thanks for the info. I'm simply trying to find the stakes on my property, but some of it is overgrown or pushed over (in some cases, there were colored surveying tape on tree branches, but cows/deer had a tendency to eat them). I was wondering if I could somehow get GPS coordinates to help me find those stakes? Looks like you answered some of my questions.
I would like to also draw the boundaries using Google Earth so I could get a bird's eye view of my property's boundaries.
I'm going to head out there this evening and see what I can stumble upon...
If you find the first stake, you could use this feature to find the next. Create a waypoint where the first stake is, then project the next one by entering the info from your survey. But here's the problem... you cannot set the disance units to feet, only yards/meters/miles. I used yards since that gave me the highest precision, but it doesn't even let you enter decimal numbers, just integer yards. In the heading field, you can only enter integer degrees, and not degrees/minutes/seconds or decimal degrees.
So this is not going to be very accurate - the farther you project the waypoints, the bigger the error. Here's what it looks like on my 60csx, which should be basically the same. This is a map I made myself, where the thin pink likes and small dots are the corners of my property. I created a waypoint and projected it using the bearing and distance from my survey. The result was the heavy pink line ending at 002. This is pretty far from the correct location.
On my 60CSx:
Projection Page, bottom right, hilite mi. then Entr. dropdown box lets one select feet, and five other scales. One can then project out up to 9999.99 ft. Sure wish the degrees would break down to min, like in the old Magellans.
You got that right!
Convert to mils ( not my favorite ).
Using the Map Page, put in a wayoint to, say like, 203 deg, and another at 204 deg, then zoom in as close as possible with both on the page, then guestimate where 07, 23, or 47 min is, knowing that 30 is half way between. Then make a waypoint there as a target goto. Usually the mils are more accurate than my guesses.
I don't know if this approach will help, but maybe worth a try. Download a GIS program called "Global Mapper". It will be a trial version, fully functional except that file saves and other certain functions are disabled, but can be used to find bearings, locations, etc. on any geo-referenced map or photo. Topos, geo-referenced aerial photos such as some state survey aerials, govt' aerials, etc. can be opened and manipulated.
I downloaded a geo-referenced aerial photo from our state archives shot in 2006 just after we bought our property. I opened the photo (134MB file!) in Global Mapper, found the point of origin of the survey using the coordinates in dec. degrees on the photo. I then plotted each metes and bounds survey point using the "measure tool" icon, and noted the dec. degree values in the lower right corner for each point, ending at the point of origin for closure. You can do a screen capture to locate the points on the photo which you then can take into the field. It's not exact, but you can find generally where the corners are for a more "precise" reading from your GPS. A metal detector is good for locating metal stakes where they are noted on the survey. Another program for plotting metes and bounds that I have used a lot is "Map Draw" for plotting metes and bounds. You can download an evaluation copy of that as well.
The whole thing has been a confusing mess. I spent a good deal of time out there on the property today, but got sidetracked by nature...had to walk down to the river that abuts the eastern part of the tract and take in all the sights and sounds. Found out that good leather boots are waterproof, so I was able to cross the low river and get to the other side of the property so I could sit in the shade and study my plat map.
Later on that day I did find stakes on the ground for some sections of my parcel, but there were also stakes not listed on the map. I also found rebar and some wood pikes lying and hidden in the tall grass, but I have a hunch it may have moved.
Lastly, I did find an old eTrex Legend unit stashed in a glove compartment...but getting that thing to locate satellites for the first time in over 10 years was a bear, even with WAAS enabled. I put fresh batteries in and the eTrex still died on me after 5 minutes of use. There was weird digital artifacts going on...like half the screen was loading and the other half was showing no pixels, just blank.
Good news? Some of the numbers on the plat map DID make sense, especially the 270.00'03" 305.25' numbers discussed earlier. What little I was able to use the eTrex for, that did ring up with the compass and distance tool.
Now the question is...how to read the Curve data...
Btw, I did find some kind of steel object near one of the survey points, but I'm not sure if it's an actual object used specfically for surveying...or it just happened to be in the same spot. Anyone know what this is ?
Sorry for the late reply, but I had trouble finding the link I had intended to send regarding land surveys.
The link is primarily a genealogy site for working with land surveys, deeds, records, etc. but has a lot of very informative info on various types of land surveys, systems, plats, etc. I have worked with land surveys quite a bit and found lots of things I was not aware of. Give it a look.
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