Dense topographic data for a lot.

January 18, 2013 12:41 PM

I've got Home Designer Architectural 2014 and a desire to buil a house on a steep hillside. I need topographic data for this.

In this program, I can import data about the topography of a lot in the form of a .gpx file. Google Earth can export a map of pins (but not a shape) to a data file that can be converted to .gpx, but placing those pins manually at a useful density would take forever and I'm not sure that Earth has sufficient elevation resolution.

So where can I get this data? Are there USGS maps with high enough resolution? An app that can plot X, Y, and Z coordinates while I walk around the lot?

Ideally I'd like everything to be accurate to within a foot, but I'll take the best I can get.

posted by cmoj to grab bag (12 answers total)

Sketchup can import Google Earth topography (File > Geo-Location > Add Location..., and then turn on the "Google Earth Terrain" layer), although it does not appear to be able to export a GPX. do you have any other format options? Sketchup can export DAE, KMZ, 3DS, DWG, DXF, FBX, OBJ, WRL, XSI. (this is with Sketchup Pro. I can't confirm if you can do this the free version, since I have the Pro version...)
posted by misterbrandt at 12:59 PM on January 18

You probably want the 1/3 arc-second National Elevation Dataset data. It probably doesn't have the 1' vertical resolution that you want. The correct (and expensive) way to do this is to get the lot shot with a LIDAR flyover, but that's likely thousands of dollars. A cheaper way would be to buy/borrow a GPS with a barometric pressure sensor, calibrate it to a known elevation when you're going to use it, then trundle it about your site at a fixed height letting it record a GPX trackpoint every second or so. This will give you about 2-3' vertical accuracy, and 10-20' horizontal uncertainty.
posted by scruss at 1:08 PM on January 18 [1 favorite]

There are apps that will track your walkabout via GPS that can record lat/lon/elevation and then can export that as a KML file. The problem is the resolution probably won't be what you want.

One-foot resolution, to me, means you want a surveyor to do the lot and give you the data for the lot.
posted by k5.user at 1:10 PM on January 18

Is there any sort of building or development on the lot already? Could you possibly check the records for that building at the building department to see if they include a topo? Has there been any development in the nearby area where a topo has been produced that includes your lot that you could buy the drawing off the firm that did it?

A USGS map would include topo information but probably not at 1' intervals. Your city may have topo maps that go down to 2'. I assume this is in Denton, TX? The city website has a GIS aplication that might be useful, but if you go down to the city and look in their maps division they might have more accurate stuff.
posted by LionIndex at 1:13 PM on January 18

You're going to have to derive this data yourself, and unless you've got a serious budget for this, you're probably best-off using old style surveying techniques.

The USGS National Elevation Dataset (NED) has a resolution of about 30 meters, and in order to get foot sort of resolution out of GPS you want you'd need a differential GPS unit with a basestation on-site that has had a month or three of dwell time.

To do this on the cheap to that sort of resolution, I'd go out with several tall (ie: long enough to account for the slope, like 16' 2x4) stakes, set those out the corners, use a hose level to mark "level" heights on all of the poles, and then run string between 'em to lay out a grid. Get another tall stake, staple a tape measure down the side, and walk the grid, measuring to your reference height on your stakes (or just carry around one end of the hose level and have an assistant record the height of the water at the other end as you record each position.

Might be helped by a laser level and working fast at dawn or dusk.
posted by straw at 1:15 PM on January 18

For the GIS: under the drop-down menu for "map" there's a content link that will pull up a checklist of information you want to show on the map. Check the "contours" box and you should be good to go. If you zoom way in, it gets down to 2' contour intervals.
posted by LionIndex at 1:16 PM on January 18

Chances are your local town government has this data. They will sell you this data for a nominal fee (not cheap probably, but way cheaper than getting a surveyor to do it for you). I work for one of these small town governments (and the bigger the town more likely they have it) and it takes about one hour or less for they're GIS (or engineering) department to pull it together for you. you will have to pay for that time, I bill out at about $65 bucks an hour. We charge about 100-200 for nice full color printed maps with this data, we let the digital copies for much, much less. So anyway, call you local Public works department, explain what you want and ask to talk to their GIS guys if possible. Chances are they will be really thrilled and you will get more advice than you bargained for. We love, LOVE talking to a reasonable, responsible property owner wanting to do good development. There are so FEW of you out there...

and BTW the data is accurate for 1/10th a foot that we use...so the best data at a resonable price...hopefully. Sometimes you get a dumb bureacrat who doesn't give a shit, but just plugging away at it. If the government isn't helpful talk to a local mid size (say 10-50 people) engineering firm for the same data, which they probably already have. It will cost a little more but they will happy to make a couple of hundred dollars for a few minutes work.
posted by bartonlong at 1:47 PM on January 18 [1 favorite]

and BTW the data is accurate for 1/10th a foot that we use

It strikes me that surveying an entire county (or even city) such that you have accurate elevation measurements to just over 1" resolution wound cost, at least, hundreds of millions of dollars.
posted by tylerkaraszewski at 1:53 PM on January 18

Is this in DFW? The North Central Texas Council of Governments sells aerial survey information of the entire region. The latest data available is 2' contours from 2007. I use this information for earthwork estimates on highway and airport projects all the time. It is an order of magnitude more accurate than what you export from Google Earth. In my experience, it matches up with LIDAR and ground survey pretty well. $10 well spent.

If you are really considering building a house, you might go ahead and hire a surveyor. The cost will be a miniscule portion of your budget. My wild ballpark estimate is $1500 per acre. You don't need to fly LIDAR for a average size house lot. Ordinary ground survey will be fine. These guys have done good work for me in the past, but I don't know if they take on jobs this small.

I have done my own surveys using waypoints on a hiking GPS. This method isn't terribly accurate and I have access to professional software to process and convert the data, so this may not be the best option.
posted by Uncle Jimmy at 1:54 PM on January 18 [1 favorite]

I have a tool that can convert a good number of formats to .gpx.

There is a building on the lot, but we're not sure if a topo was done. I'll try to find out.

This is in Fort Worth, TX. Sorry about the outdated profile.

Looks like I've got several options as far as doing it manually with a topo. I think 2' is good enough, I just wanted the best I could get reasonably. I'll try the Sketchup/Earth method since it's mostly automated (and free) and see if that gives me anything I can work with. Thanks!
posted by cmoj at 2:11 PM on January 18

and BTW the data is accurate for 1/10th a foot that we use

It strikes me that surveying an entire county (or even city) such that you have accurate elevation measurements to just over 1" resolution wound cost, at least, hundreds of millions of dollars.

IT is done with some kind of LIDAR arrangement mounted on a plane and then overflew. I do think there is some interpolation, not sure how much. We only use the 1 foot data for most things since that is close enough and any more is really just gilding the lily since there are tolerances and such on construction that make anymore meaningless.

I don't know the fine details, buy the data for the town (about 5 or 6 square miles) is BIG, stored on multiple drives on our GIS server. Which is why I have to ask GIS to cut out the specific area of limited size or its too big.
posted by bartonlong at 4:34 PM on January 18

My wife and I surveyed our acre building site using a surveyor's rod and an autolevel (plus walkie-talkies, clipboards, and some swear words and beer). It was not...fun, but it was doable and gave us the contour data I needed to design our house to fit a sloping site. Our method was to do a rough meter grid, plus obvious benchmarks like water well caps, etc. We then added in USGS topo data and interpolated it all together. One foot is a pretty tight grid.
posted by werkzeuger at 6:06 PM on January 18