Some of the sites I design for are flat, and some aren’t. For my land in 97, my wife and I rented a laser distance finder from a surveying shop for a weekend, and crawled all over the site measuring distance and elevation. To create 3D models with reasonably accurate terrain, I want a better, quicker, cheaper way of doing this. I have a site a couple of hours from here that is a challenge. It is so overgrown that the laser thing wouldn’t work so good. And it’s on a ridge, so big contour/elevation changes. String and a tape measure would be a nightmare. Transit and surveyor’s pole would be a real pain. Maybe everything is a pain and there’s no easy way to do this. I’ve heard of robotic devices and GPS based stuff, but don’t know the state of the art.
What’s the best way to get good data in perhaps a 150′ radius that I can use as the basis of a site model? Need precision more than accuracy–don’t care if it measures 2831.47′ above sea level. Just care if I’m 13′ above my base point or 13.5′. Is current GPS up to this?
Replies
Have you considered an aerial survey?
I don't know their precision, but I imagine the cost would nix the idea for most clients. I have the topomap, but the resolution isn't good enough for the needs. For example, I can figure out there's a rise of 25' over 110' at some point, but I need more detail than that, and only in the area around the house site.
We use aerial stuff for engineering projects quite a bit. The elevation data is pretty good (0.5 feet or maybe a little better) in terms of precision (i.e., elevation differences from one point to another). They're accuracy is usually worse than that- maybe 1 foot or so. For an area of two acres or so, the cost would probably run around $5k, so not cheap, but may be worth it.
One other thought- the USGS has DEMs (digital elevation models) available on-line. They're electronic files containing 3-d points on a 10 meter grid. I think most land development software packages can import these files. I've used them when we had nothing else to go on. When using DEMs, I did notice some significant differences as compared to actual field survey data. However, most of the sites I've worked on have been changed topographically over the years since the DEMs were generated and that may be the cause of the differences. Possibly you could locate the DEM for your area and compare it against some other type of survey (USGS quad maps for instance).
If all else fails (or maybe this should be where you start), try contacting a surveyor in the area to see what suggestions they have. I think the type of GPS equipment you need for surveying is pretty expensive.
Edited 8/21/2003 2:02:47 PM ET by stonebm
Seems to me the last consumer level GPS that I looked at (One of the top of the line Garmins, I believe) said that the best positional GPS accuracy with their proprietary augmentation was about 3 meters. As best I could determine, the altitude accuracy was something over half again less accurate, or around 5 meters vertically. Of course your mileage could vary - i.e. surrounding high hills, tree cover, etc.
I saw a Suburban pulling a trailer lately that had advertising on the side indicating they did site surveys using a small blimp. I actually bought a small digital camera and some large balloons plus a canister of helium, but never actually rigged it up to take pictures as a commercially available high altitude photograph turned out to have sufficient accuracy for my purposes. There are people who have rigged up the small model helicopters to take aerial pictures, but those things are a bit more pricey than balloons (I think one with enough payload to carry a small camera would have run around $300+ at the time with minimal radio control gear). Of course, this type of aerial photo would only give you the placement of buildings, trees, fences, etc. and not the altitude that you want.
This may help. It doesn't show plot lines but is the USGS topos available for the whole country. Easiest search is by zip code, then zero in. click for the topo version
http://terraserver-usa.com/default.aspx.
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Here's another http://www.topozone.com/map.asp?z=17&n=3943365&e=364449&s=25&size=l It's very cool, and the data is available free, but it seems tricky to convert to usable form.
Bill, "differential GPS" is a great term and the exact thing I'm looking for. Of course, I want it for about the cost of a box of cereal. :)
Slightly off subject, but you should know that I like to brag about my location, in Hawaii. The national park at Volcano uses GPS to track all the lava flows, and cracks, and swelling, and well you get the idea. They claim to get very precise results, but I imagine the equipment is expensive!! Not sure how you would find out what they use, but thought it was worth mentioning. Other way would be to clear as much of the brush as needed, but you have already thought of that!!
Dan
Accuracy is one thing, repeatability is another. I rarely see repeatability specs on GPS, but it may be sufficient for what he is trying to do.
He doesn't care if the spot is correct on the earth, just that if he walks a foot "up and to the left", it registers that change; then if he walks back, it returns to its original reading.Steelkilt Lives!
Sounds like what our site looked like when we had it shot. Our surveyor used a backpack GPS unit on a telescopic pole that got him up above the tree lines for a clear signal. It measured all the critical information at any given point where he set the tip. He e-mailed me a cad file with point elevations all over the site a couple days later. They charged by point but I don't remember the exact rate. Seems like we spent around $600 on the survey but we were splitting the cost with the land owner we bought the property from so I can't remember if that was half the total cost or all of it.
To answer your question - yea, GPS can do that real well. Check with a local surveyor.
Kevin Halliburton
"I believe that architecture is a pragmatic art. To become art it must be built on a foundation of necessity." - I.M. Pei -
Is current GPS up to this?
I've been told that large sale excation uses GPS to control height/depth/grade to sithin a fraction of an inch!?!
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Bob
I think that what they use is differentail GPS.
http://www.geography.wisc.edu/sco/geodetic/gps.html
http://www.gambaengineering.com/surveying/
The last one indicates that with the differential they can get better than 1" accuracy.
Differental GPS can be used with high accuracy in toptgraphic surveys.
However, in your case you may want to consider locating all terrain features (trees, creeks, etc.) within your 150' radius.
An EDM like you indicated that you had used before would be your best answer for the order of detail you want. You can pick off gross elevation contours from the USGS 1/4 degree plates, and couple these with your other locations.
Grid location is generally the way it's done. It'll require some brush clearing to along your grid. You can use your EDM to locate not only trees, but also get your topo shots in the kind of detail you require. Flat areas, every 25', areas of great relief, as needed. Use your rod and reflector when shooting the base of trees, and record that as an elevation too.
Use a copy of your plat to scale off a grid in the area you require, not forgetting to include road areas, areas for drain fields, etc.
Scale from the nearest known point, and construct your grid. You can use a regular transit for this if you have one. The lines only have to be enough to see through, and walk. Number your grid points on your sketch, and lath them on the ground so you can enter them in your data recorder.
Set a BM (bench mark) of known elevation as close to your topo area as you can. Flag it so it won't be destroyed during construction.
CloudHidden-
What are you using the data for - site design and dirt quantity take-offs? Aerial surveys tend to be pretty accurate as another post indicates, I would say .5 foot maximum deviation across a site with an average .1 deviation is pretty fair of their accuracy. My experience is to usually drop an aerial topography file .2, that seems to give pretty accurate results. If you're doing site design you don't need more accuracy than that, the best finish dozer operators get to .1
Why wouldn't you want SL elevations? are you staking your own job sites? The GPS stuff has too many problems. You can walk it on a back-pack unit, and I get a topo bust of 3 feet where the guy bent down to tie his shoe. the ATV GPS stuff is crap too, every bounce in the suspension is +/- .2 on the file it generates. Use aerial files.
remodeler
>site design and dirt quantity take-offs?
Yup. It's a tricky site with a tricky stepped footer. I don't want to design the footer to be on fill dirt or hanging in mid-air off the side of the ridge. It's here http://www.topozone.com/map.asp?z=17&n=3909487&e=233528&s=25&size=l , near the little red cross, but a little higher up the ridge...toward that flat area at the 2800' line, though when you're actually there, it sure doesn't seem none too flat. That's why I'm looking into more precision. I've attached an early crude model from the contours I pulled off the topo. The site is COMPLETELY overgrown. Extremely dense forest with a thick canopy. How does this affect aerial work?
Part of my question is for this project, but part of it is to just update myself on the state of the art for future projects. Budget is also a factor. There's always limits to what costs the HO can absorb--of course they want their money going into things they can touch, rather than prep work--and I cannot just eat the cost. So, searching for best ways and means to accomplish the job.
Wow. Looks like a beautiful site - 200' contour lines that close together would scare me to death. The accuracy of the topo with such overgrowth depends on the surrounding area and the climate. I'm by no means too knowledgeable about surveying, but I see the mistakes they make all day. Here aerial topos are usually shot in winter to minimize distortion from vegetation, etc. Also clear areas like a meadow down a steep slope make them more accurate.
Is your site sheet draining to the two valleys shown on the USGS topo, or do you have more complicated terrain around your building site? That looks ideal for walk-out basement stuff. A site like that would be wonderful for me, we'be been building walking-out basement sites out of clay and it is always a big pain in the *ss because of how much clay it generates. Are you doing a walk-out, or is it a cut/fill for a slab?
remodeler
btw it's a slow day here in Indiana site-work wise.
Drains to the rivers/creeks on each side. The front-most dome will have a walk-in basement....actually, a drive-in garage. The only cut and fill will be from there to the car level. The upper two domes will have excess dirt off the sides, but only to provide a walking area around. I want every footer on original dirt.
Cloud, how about getting the main part of the site cleared before you establish existing grades (regardless of the method used). I realize that you may be trying to keep certain trees or other vegetation, but if you know pretty well what the customer wants, you could have at least the underbrush taken out now. A lot of that vegetation will have to go no matter what, right?
I used a dumpy level on our 150' x 150' site to create a grade map. Started at the bottom, set a temporary reference rock, then shot all the high spots and low spots I could from that position, then moved up and shot again, etc. I did all the math to create the map, then went back and rechecked the main points to be sure I wasn't off somehow. It worked out fine here, but of course my site is not as big or as overgrown.
I know a surveyor who has the GPS equipment to spot a gnat's arse, but he told me that the field equipment was $30,000 plus software!
If you think about going the route suggested by Kevin, check with a local surveyor about prices. I've sold a number of pieces of Colorado land to Texans and they always think that surveys here are way too expensive. I guess for some reason surveying is inexpensive in Texas, or maybe it's just expensive in Colorado!
How about posting the solution that you finally settle on? I find this to be an interesting question.Any jackass can kick down a barn, but it takes a carpenter to build one.