My wife and I have a nice chunk of land in the Blue Ridge mountains. I’m drawing up our dream cabin now, but have no idea how to plot the actual terrain onto paper or scale model. Google wants to sell me terraserver pictures… not exactly what I’m looking for. What’s the dope on building a scale model of some sloping/uneven footage?
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Youi might want to check out topozone.com
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As a condition to getting a building permit, the building department of your local municipality may require a grading plan, with contours at one foot or two foot intervals. It’s something that your builder will take care of, usually through subcontracting a surveyor.
How accurate do you need to be? A quick and dirty lay of the land could be had by walking the lot with a GPS unit and recording random LAT LON and Elevation points.
I personally would then take the data into a 3d rendering program such as 3D Studio or Autocad. Add in some simple models of the main buildings and you should have a nice visual representation.
If you decide to stick to paper, and are familiar with topo maps, you could again use a gps to walk elevation lines. Start at the bottom most corner of your lot (assuming rectangular). Using your handy second generation GPS unit, walk uphill along a boundry line until the elevation reads a nice even number (say 2010 feet). Then, start walking across your property staying as close to 2010 feet as possible. When you reach the edge of your property, walk another 10 feet uphill along the edge, then head back across the property. When you reach the opposite corner, you should have a decently accurate topographic map.
To turn into a nice model, get some of that grade school art project board stuff (forget the technical name) and cut out a slice for every elevation line you walked. Stack em, glue em, and you’re done.
Are GPS elevation figures really that accurate. tastycorn? Most hand-held GPS units that I’ve seen use barometric altimeters, precisely because GPS is so inaccurate for elevation. I very much doubt it would be any use at all for recording 10ft contours.
You could always buy a USGS Topo map.
But are you building a model or are you making a map? You could rent your own surveying stuff and do it yourself, it takes only two people and it’s not that hard once you get it down. I just don’t remember the formulas but just using a few base stations you should be able to do 1 foot contour lines no problem.
GPS is pretty much useless for that. I can tell you my boat is always right at 0’ elevation but the GPS some days reports it as a submarine 100’ under the surface and others as an airplane flying low (and very slowly). You can do a rough plan yourself with just some basic tools or have a survey done.
There are several places on the Net that have really good scans of USGS Topo maps.* Unfortunately, I have yet to find a good site that links to them all. Most sites cover just 1 state or a region. These are a good start to get reference elevations in or around your property. Google terms like “USGS Quad DRG (Your state)” and such.
Topozone is handy for lower res. stuff that covers a lot.
According to my local quad, I’m sitting less than 10 ft in elevation below the nearby Eastern Continental Divide.
*US Govt publications like these are not copyright and can be freely copied. The scans might be considered IP, but the ones posted to the Net by the scanning organization are clearly unencumbered.
It all depends on how detailed you want it.
If you’re wanting a very detailed map of your particular lot, the best thing to do is call up one of your friendly local surveyors and have him and his crew do a topo survey of your lot. They’ll give you a spiffy map with the contour lines and everything based on their survey, and probably the AutoCAD file with the COGO data plotted in- in other words, it’ll be a 3d autocad file. At least that’s the way we did it back in 1993 or so! I imagine you could render it with AutoCAD’s rendering tools, or ask the surveyors to for you.
The second generation GPS units can be very accurate indeed for elevation, providing you have more than four satellites being received at once.
I’ve done quite a bit GPS caching, and on a good day, my Garmin will track you walking up stairs. You can walk a 5 foot circle around a nice cache and have the GPS point to the exact middle of the circle.
Keep in mind, you are not walking one elevation one day, then coming back two days later and walking the next. You are just using it to record variations, not absolutes.
Its not perfect, but it does work. Keep in mind, the suggestion was for a visual or rough planning tool only. If you need exact, then there is no way around a survey.
I think even with WAAS, a GPS unit is unlikely to be accurate enough in the deep woods. I’ve also done a lot of caching and in heavy tree cover, I’m lucky to get within 30-60 feet of a cache, to say nothing of altitude.
I agree that the thing to do is to see if there’s a USGS map first and if that’ll take care of your needs. Best place to look for one of those would be a library, either public or college.
Tree cover could be a problem. I get so used to the lack of real trees in SoCal I tend to forget they exist. Do they have palms trees in the Blue Ridge Mountains?
Like I said before, GPS doesn’t do that well with absolutes (even WAAS, as you pointed out), but for relatives (which is what surveying is based on anyway) it does a decent job.
I have a friend who spent several summers working for the national park service making topos. How did she do this? Walking elevation lines with a GPS unit. I have also personally used this method to fill in detail for topo maps of our ranch with good results.
Accuracy depends on equipment as well as reception, but it does work.
I’m afraid a standard handheld GPS is not going to be anywhere near as accurate as you need. Accuracy of your basic “out-of-the-box” units usually ranges anywhere from 15 to 50 feet…and that’s a lot of error for precise plans of the sort you need. And, as some have pointed out, tree cover can make GPS accuracy head south fast.
In addition, unaugmented GPS units have a notoriously difficult time reading accurate elevation, and I suspect that in the mountains, you’d want a fairly high degree of 3D precision.
There are a couple of augmentations that are available for GPS. One is the WAAS transceiver. Essentially, it acts as a remote ground station, a known point from which to triangulate your position. The WAAS station nearest you transmits a signal detailing its position to a high degree of accuracy. Excellent system, but there are a couple of problems: 1) You’re still not going to get sub-meter accuracy with it. Count on 1-5 meters, but that’s about as close as you may get. 2) WAAS has a few “dead spots” in the nationwide coverage. I don’t remember right off if the NC mountains are in there, but I know that a couple of dead spots are in the general vicinity. 3) Expense. You’ll spend a pretty chunk of change on a WAAS unit ($1000 or more.)
There is also the option of having a ground station. This is a unit that you set up on site to provide a known, unmoving point. This WILL give you sub-meter accuracy, no question. The problem is that they are very, very expensive.
All of this aside, I believe you have a few options for surveying the land. First, you could hire someone. Second, you could get local geography grad students to come out and do it for free. Hey, it’s a project, and I’d do stuff like that at the drop of a hat in grad school.
As far as existing maps go, the standard USGS 7.5 minute quads may be perfectly acceptable. You should also check with your county planning office, since I know that some counties (in Tennessee…not sure about NC) have commissioned very-high-resolution topos at 1:10,000 or better. You may also have some luck with a GIS approach. 1:24,000 digital elevation models of North Carolina exist, and paired with 1:24,000 digital raster graphics, you could easily build a flythrough model of the land. As always, of course, there are data entry and conversion problems to overcome.
If I knew more about the size/location of the land, I might be able to help you more. For what specific purpose are you going to put the information? Will it be a planning tool, or will it need to be a fairly precise “OK, this is where I should sink the support beams” depiction? If the former, it shouldn’t be very difficult. If the latter, have that son-of-a-gun surveyed!
http://www.jpl.nasa.gov/srtm/index.html may help
Brian
Thanks, all, for the dope. We’ll hire a small land-based crew shortly to do the dirty work. Seems to be the simplest (and obvious) way to do it.
Terra Server is free, its proably not detailed enough for you stockton, but fun to look at. MS owns it so you can only see it with IE, as I just tried it.
http://terraserver.microsoft.com/