Can new technology help me map my underwater fieldsite?
April 29, 2013 11:47 AM Subscribe
I need to make relatively precise measurements, to the nearest half meter, of locations on my field site, which is on the floor of a Mediterranean cove (not more than 10m deep). One possibility is to swim on the surface above my locations and use a GPS to record the points - but so far my older-model GPS's have not been accurate enough for this.
I'm leaving in about a week, and just recently I was reminded that technology may have advanced to meet my needs, even since last year! Can anyone update me on the latest and greatest (waterproof) mapping instruments out there?
Side scanning sonar? What are you measuring?
posted by oceanjesse at 11:55 AM on April 29, 2013
posted by oceanjesse at 11:55 AM on April 29, 2013
(GPS is more accurate than 10m resolution, btw, but only for the U.S. military, and with special equipment)
posted by sexyrobot at 11:56 AM on April 29, 2013
posted by sexyrobot at 11:56 AM on April 29, 2013
Best answer: The industry standard for bathymetry is sonar or LIDAR. The method you are proposing is not going to be very accurate or defensible for the floor topo, depending on what you need.
However, I have used a handheld GPS unit (trimble) on an anchored barge to get points without floor altitude (matched it up with the bathy model for that), and that was acceptable to peer review. You just need an accurate GPS unit and a reasonable anchor to keep you from drifting off the point. We put ours in an otter box to waterproof it!
posted by cakebatter at 12:00 PM on April 29, 2013
However, I have used a handheld GPS unit (trimble) on an anchored barge to get points without floor altitude (matched it up with the bathy model for that), and that was acceptable to peer review. You just need an accurate GPS unit and a reasonable anchor to keep you from drifting off the point. We put ours in an otter box to waterproof it!
posted by cakebatter at 12:00 PM on April 29, 2013
Response by poster: I'm measuring nesting sites - so the bathymetry isn't as important as the relative position/distance between the nests.
posted by seachange at 12:02 PM on April 29, 2013
posted by seachange at 12:02 PM on April 29, 2013
Best answer: A commercial GPS can get you sub-5m or sub-3m resolution using free WAAS, EGNOS, or MSAS augmentation (available around most countries including some of the Mediterranean). If you've got the cash you can pay for a commercial satellite-based augmentation service like VERIPOS or OMNISTAR to get sub-1m accuracy. RTK hardware and services can get you to centimeter accuracy. You can rent these sorts of things.
Commercial augmentation systems have made GPS ridiculously accurate compared to military P-code GPS. The trick with P-code is that when they turn off the civilian GPS frequencies, ONLY the military has positioning. The idea that only the military can get sub-meter accuracy is not correct. There are entire survey industries that rely on the fact that this sort of thing is commercially available.
posted by olinerd at 12:04 PM on April 29, 2013
Commercial augmentation systems have made GPS ridiculously accurate compared to military P-code GPS. The trick with P-code is that when they turn off the civilian GPS frequencies, ONLY the military has positioning. The idea that only the military can get sub-meter accuracy is not correct. There are entire survey industries that rely on the fact that this sort of thing is commercially available.
posted by olinerd at 12:04 PM on April 29, 2013
Do you have any local university or gov contacts there who could loan you a nicer GPS unit? Or if a lot of research is done on the area, maybe a local geodetic system to calibrate to?
posted by cakebatter at 12:06 PM on April 29, 2013
posted by cakebatter at 12:06 PM on April 29, 2013
Best answer: Presumably this is a nearshore setting. You should be able to set up a differential GPS base station on land which can get you accuracy well under half a metre.
posted by pont at 12:19 PM on April 29, 2013
posted by pont at 12:19 PM on April 29, 2013
What about placing some colored stakes/flags/balls on sticks on the shore, registered to some map?
Then wave a waterproof camera at them from your location?
If you have a photo of a rosette of stakes at point 1, you may get a decent angular fix from the relative position of various poles. "Ok, center pole lays between the North pole's stake and the 15 degrees east stake. I'm looking rougly 5 to 10 degrees east of geographic north." And then looking at point 2 (and 3), you get a second angular fix, which allows you to triangulate your point on a map.
Here I wait for a surveyor or civil engineering student to say how to do it properly.
posted by sebastienbailard at 12:58 PM on April 29, 2013
Then wave a waterproof camera at them from your location?
If you have a photo of a rosette of stakes at point 1, you may get a decent angular fix from the relative position of various poles. "Ok, center pole lays between the North pole's stake and the 15 degrees east stake. I'm looking rougly 5 to 10 degrees east of geographic north." And then looking at point 2 (and 3), you get a second angular fix, which allows you to triangulate your point on a map.
Here I wait for a surveyor or civil engineering student to say how to do it properly.
posted by sebastienbailard at 12:58 PM on April 29, 2013
Taking a GPS fix from the surface requires that you hold the antenna of the device directly over the site that you want to map, and hold it there long enough to get a good fix. As suggested, DGPS will greatly reduce that time needed for that fix.
However, the primary problem is still one of holding an accurate position directly over your site. Wind, tide, and actual motion of waves will conspire to push you around. Tides and wind will induce a steady offset that you have to combat by swimming, and waves (depending on their wavelength) will move you rythmically back and forth, which would require time to average out when you were getting a fix.
Cakebatter's barge did position averaging physically, with inertia.
If you want to use new technology, I'd suggest dropping one of these passive sonar reflectors at a controlled distance and vector from each nest, and to roll over the whole thing with some sort of sonar device, while mapping.
How far apart are your nesting sites? How many are there? If they are dozens, say, within a hundred meter square, I'd be tempted to set up a cove-floor survey grid, with several repeatable landmarks, and metal tapes to measure the distance to two landmarks, to triangulate the position of each nest. You would need a helper.
posted by the Real Dan at 1:44 PM on April 29, 2013 [2 favorites]
However, the primary problem is still one of holding an accurate position directly over your site. Wind, tide, and actual motion of waves will conspire to push you around. Tides and wind will induce a steady offset that you have to combat by swimming, and waves (depending on their wavelength) will move you rythmically back and forth, which would require time to average out when you were getting a fix.
Cakebatter's barge did position averaging physically, with inertia.
If you want to use new technology, I'd suggest dropping one of these passive sonar reflectors at a controlled distance and vector from each nest, and to roll over the whole thing with some sort of sonar device, while mapping.
How far apart are your nesting sites? How many are there? If they are dozens, say, within a hundred meter square, I'd be tempted to set up a cove-floor survey grid, with several repeatable landmarks, and metal tapes to measure the distance to two landmarks, to triangulate the position of each nest. You would need a helper.
posted by the Real Dan at 1:44 PM on April 29, 2013 [2 favorites]
Response by poster: I'm doing the cove-floor survey to triangulate each location now. Experience has shown it is very time consuming and not very accurate (because the site is so rugged). I will have to measure 100-200 locations.
Thanks for the suggestions on DGPS and the augmentation system suggestions. Seems promising.
posted by seachange at 3:06 PM on April 29, 2013
Thanks for the suggestions on DGPS and the augmentation system suggestions. Seems promising.
posted by seachange at 3:06 PM on April 29, 2013
Depending on the maximum distance you need between points you can get a laser rangefinder that interfaces with Trimble GPS units. You can have a person stand on the shore with the Trimble and the rangefinder and measure the distance and bearing to someone (perhaps holding a target) at each best location. This method would keep the GPS dry.
posted by buttercup at 6:39 PM on April 29, 2013
posted by buttercup at 6:39 PM on April 29, 2013
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posted by sexyrobot at 11:54 AM on April 29, 2013