O Log - More on LIDAR

    [ 1-May-08] 

      Well, I can probably count on one finger the number of people who both read this page and are interested in LIDAR base maps, but Eddie sent me this note in response to my whining about the LIDAR at Rosaryville. I want my minutes back for the time lost on #9!.

      Hey dude,

      I just read your log regarding rosaryville. I've been playing with the original lidar data that they used - Greg Lennon sent me a piece of it. I have to tell you that this is rather poor lidar data, and its likely that Price Georges county got screwed by the contractor. The reason Jon couldn't map the gullies from the lidar data, and the reason you (correctly) felt that the contours were soft is because the lidar data was apparently collected leaf-on. Greg had to smooth the resulting bare-earth data to make the contours because there was so much leftover vegetation corruption of the bare earth. That is, because of the leaves there were very few ground returns even recorded. Have a look at some images I made on this web page:

      http://www.catchingfeatures.com/eddie/rosary/page1.html

      Have a look at the top two links. First one is the "bare-earth" lidar data that the contours were made from. This has been filtered to remove the slowly varying elevation changes, leaving behind only sharp features - somthing called unsharp-masking. The upper right corner of this image is the NW corner of the O map. You can see the flooded lake that we went around and the forest to the N of it. Notice all the crinkly looking things in the forested area. This is 1m posting lidar data - normaly very good, but in this case you can't make out much of any detail on the ground. Now check out the lower left of this image. There are some cornfields down there and you can actually see places where deer have walked through and bedded down. This is a hint that the data was collected early in the season. As it turns out the data was collected in October. The second link there is the google maps image of the same area, which in this case is leaf-off and there is nothing standing in any of those fields.

      Now have a look at the last link on that page (fr_lr_compare.jpg). The top two panels show a piece of the lidar from near my apt. Left is first return and right is last return. In this case, the last returns are indeed mostly reflections off the ground, as the laser pulse first reflects off of high branches, and then hits the ground below. Note that the powerlines and most of the tree canopy is gone in the last returns image. The only thing remaining are the large tree trunks and some thicker understory. All of this remaining stuff can be easily filtered to leave behind very clean bare-earth.

      Now look at the bottom two panels. This is the same first-returns left, last returns right, but from the Rosaryville data. Almost no difference between the two. The laser is simply not penetrating to the ground.

      So now when you try to filter the remaining vegetation, there is almost no ground information to work with. You end up throwing out nearly every point, and thats why the bare ground in the Rosaryville unsharp masked image looks so ratty. Greg had to smooth pretty heavily to keep all those jaggies from corrupting the contours, which made them softer. Also might explain the higher than expected #9.

      The third link on that page shows some bare-earth unsharp maked data from the Mckeldin map that Ted is working on. Left is 2m res lidar, right is 1m res. Both are under full forest canopy. This image covers about 700m from top to bottom. Note the level of detail in both. Vegetation filtering was excellent in both cases (was leaf off). The 1m data is just exquisite, with pits, faint trails, ditches etc easily identifiable. The Rosaryville data was also 1m, so should have looked very much like this if it had been taken leaf-off, and Jon would have had not trouble at all mapping the gullies right off the lidar.

      Anyhow, this is something I've always been worried about. People using lidar blindly without minding the limitations of how the data was collected. There is good lidar and bad lidar, just like any other remotely sensed data. THis is about the worst I've seen. So please don't lose faith in it :) Ted's new map is excellent (his first one), and the new Prince William map used for the US Champs also had very good contours. Greg is going after PG county now to see what gives and hopefuly we can get Jon some better data to fix the Rosaryville map. It seems like a pretty nice area.

      Oh, and here is a nice dataset of Great Falls in D.C. Just found this last week. Pull out an old map and see what you can identify:

      http://www.catchingfeatures.com/eddie/gf/page1.html

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