Thursday, February 27, 2014

Revealed in LIDAR

I was hoping to get out and compare LIDAR images to the actual sites, especially since I have some estimated GPS coordinates.  Unfortunately, winter is hanging on.  There's 6 inches of crusty snow on the ground, and more on the way.  In the meantime, here are some images I have been working on.  Looking at  LIDAR images of familiar places reveals how different types of terrain appear in LIDAR.  This is Johnston Town Forest, looking north from Hartford Ave. I-295 is to the left.

The Town Forest always seems like a wilderness when I go there, but here it looks tiny, squashed between the hill and the highway. Stone walls from farm fields are visible on the hill in the center.  Most of the forest is swamp and marsh, and these appear as very grainy areas colored blue. In the lower center, those "blocks" in the water aren't ice floes, but probably LIDAR artifacts from reflections off the water.
     One of the big advantages of the Grass GIS map viewer  is that images can be tilted or rotated to be seen from different directions or perspectives.  Here's an overhead view of a site in RI.

There's a reservoir to the left, houses along a main road, and walls around a field at upper right.  Here's the same image tilted.

There is more detail in the foreground, and the distant field walls are hard to see.  I can make out two sets of farm walls in the foreground.

These images can be enlarged in the Grass GIS viewer, but the files are so massive, the process is slow and prone to freezing.  Smaller areas can be cut out of the image file in Fusion, and exported to Grass GIS.  Here are the farm walls at lower right.

I see a cellar hole and three long walls.

Dirt roads and foot paths also show up very clearly in LIDAR, and I marked them with broken lines.  Aerial photographs from the 1940s show this area covered with forest.  This farm has been abandoned for at least 100 years, and the path to the house is still visible in LIDAR.  There's no trash or fire rings in the area to suggest recent use.  A foot path indicates there's something important at the end, which may be worth investigating. Foot paths also indicate long-term use of an area.  There's a famous cairn field in which foot paths shown in LIDAR seem to branch off to some individual cairns.  This would indicate frequent visits to the cairns, whether to build them stone by stone or to visit them.  Further studies will have to wait until the snow melts.

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