GPS for informal survey of my yard - not a boundary survey.

[tashirosgt]

Give me a recommendation for a GPS unit. Here are the requirements:

I want to make an informal survey of my yard in Las Cruces NM USA, to plan some yard work. My plan is to use a laser level, detector, and surveying rod to measure the elevation data (the z-coordinates). I'd like to use a GPS unit to measure the (x,y) coordinates.

This is not a boundary survey, so I don't need much absolute accuracy. (For example, itt doesn't matter if the corner of the front porch has coordinates (3 ft, 2 ft) one day and ( 13 ft, 12 ft) the next day.) But I would like to get the relative accuracy of the locations down to a few inches. (For example, if the distance between the corner of the porch and the edge of the step is 11 inches on one day, I'd like it to be about 11 inches the next day)

Other desirable features would be the ability to type-in the elevation at a location and store the coordinates, so I could later read them or download them to a PC.

zI'd like a display that can be read in daylight, out of doors. I don't require a map display. it would be a nice extra feature if I could draw and input a map of my yard from my PC. However I want a hand held unit. That's a more important consideration than displaying a map.

[Tim]

[Boyd]

I have tried "surveying" several pieces of land I've owned over the years with as many as 5 different consumer GPS units and would have to say that it's pretty much a waste of time unfortunately. The accuracy just isn't there.

You say "yard" which implies something smaller than an acre. You are really going to be banging your head against the wall in that case. If you had 50 acres and just wanted a rough idea of the boundaries then it might be useful. But not for something small.

I think you are making some faulty assumptions here: "But I would like to get the relative accuracy of the locations down to a few inches. (For example, if the distance between the corner of the porch and the edge of the step is 11 inches on one day, I'd like it to be about 11 inches the next day) ". That's not going to happen.

Every consumer GPS I've seen specs accuracy at about 10 meters. Picture a circle with a 20 meter (66 foot) diameter and the corner of your porch is at the center. If you mark a waypoint here on different days it should fall somewhere inside that circle each time. But worst case, the points could be at opposite ends of the circle - 66 feet apart - and the GPS would be operating within specs. Of course that's a worst case scenario, but I've done this a lot and can tell you that readings taken at the same point on different days/times will frequently be 20 or 30 feet different.

I think you will be better served by spending $20 on a 300 foot tape measure for a project like this. Otherwise, be prepared to pay anywhere between $2,000 and $6,000 for a professional grade GPS which can give you sub-meter accuracy.

[tashirosgt]

Thanks for the advice. Yes, my yard is less than an acre. It was interesting how un-rectangular the house measured.

[Boyd]

You might want to see what kind of digital orthophotography (translation: aerial photos) is available for your area. Here in NJ the whole state is covered with very high resolution imagery - one foot per pixel. Armed with this and your survey, you can make a pretty accurate map of your property, if that's what you want.

You just need the right software. You can download the free trial of Global Mapper here: http://globalmapper.com/ . It can do just about everything but export files. Then you need to dig around your states GIS website. There should be a place where you can download the data. For New Mexico, Google found this as the first hit: http://rgis.unm.edu/data_entry.cfm

Global Mapper can open almost any kind of file. Typically the data you download from websites like this will be in a .zip archive. Just tell Global mapper to open the zip file and it will probably figure out what to do all by itself.

[tashirosgt]

Now that your reminded me, I think that there are aerial photos of city that one can get cheaply from city hall. That would be OK if I only measure elevations at landmarks that I can see on the photos. My yard is fairly simple and, if the truth be known, I have plenty of old fashioned surveying equipment since my father was a surveyor. I can stake out a grid. But I'm interested in trying out new technology.

It seems like there should be a technology where a surveyor sets up low power radio transmitters at known points and uses a receiver to determine where other points are. It wouldn't involve satellites at all.

[Boyd]

Yes there are GPS stations you can put at known locations to do exactly that. How many thousands of dollars did you say you want to spend on this project?

No need to purchase anything to get hi-res aerial imagery. Use the link I posted above, it is a free download - see:

http://rgis.unm.edu/loader_div.cfm?new=true&theme=Digital%20Orthophotography

Just a quick look shows that these are mostly 1 meter / pixel resolution. You can also download this sort of thing from the National Map Seamless Server for free. Terraserver also has hi resolution urban imagery which is free and can be accessed directly inside the Global Mapper program.

[tashirosgt]

I've actually browsed the web for surveying stations, but they aren't what I described. Those stations receive GPS satellite signals and then make corrections. What I'm talking about wouldn't involve satellites at all. It would depend on putting at least two low power transmitters at known points in the yard.

Another technology would be to use a laser as a range finder and locate points by range and angle from a known point. I gather that's what modern laser based surveying transits can do.