Topic: More blurred image questions
I'm trying to use COSI-Corr to look at movement along slow moving landslides in Central California.
I have NAPP aerial photographs (scanned at 14 microns) from 1989, 1994, and 2002. I also have a LiDAR DEM (gridded to 0.5 meters, flown by NCALM for the EarthScope project) which covers a portion of the study area. I've been using a hillshade from the LiDAR as a master image to set ground control points and rectify the aerial photo. I've typically been using 30 GCP for my orthorectification.
I've been running into some problems which I think are due to the fact that my LiDAR does not completely cover the area I'm interested in using COSI-Corr over.
Here's a snapshot of my DEM:
The landslide I'm interested in is ~90% contained within the LiDAR data, with just the toe cut off. In order to avoid problems with outliers/negative values/etc, I've set all the no data points in my DEM to approximately the mean value of the DEM.
When I attempt to resample my NAPP image using the mapping matrices from the full extent of the LiDAR above, I get a rather blurry image:
(note the hdr file for my resampled image has the line: Kernel: Sinc Sinc Size : 25 Distance Max dx=31.4428 dy=10.5500)
I've done this same procedure on other nearby landslides and had only slightly blurred images:
Here's the other DEM:
And the resulting resampled image (Kernel: Sinc Sinc Size : 25 Distance Max dx=3.1748 dy=5.8180):
So, I'm not really sure why this time around it turned out significantly more blurry than previously, but (following instructions from the above post) I attempted to solve the bluriness problem by computing the resampling distances on a subset of the mapping matrices. This worked to create a much crisper image (dx=1.8044 dy=1.1025), however I noticed that the areas of the image within the extent that I selected for the the subset of the mapping matrices seems to be better rectified than other parts of the image (I've observed trees on stable ground offset by 1-5 meters between the LiDAR and resampled image). Given the geometry of the landslide I'm looking at, the extent of the LiDAR, and the fact that (as far as I can tell) one must use a rectangle to define the subset area, I'm not able to choose a subset such that the entire landslide area and immediate surrounding terrain is included within the subset.
So I guess I have a few questions:
1. Does it seem likely that the blurry resampled images are due to the inclusion of no data regions (replaced by mean values of elevation) in the DEM? (This is what I've assumed is causing the problem, but I don't really know)
2. If so, is there a way to define a polygon as the subset area to compute the resampling distances over instead of a rectangle. This would allow me to select the entire landslide without including any no data values.
3. Does it seem likely that the offset I observed in my non-blurry resampled image is due to the fact that it's outside the spatial extent of the subset of the mapping matrices selected? Or are there other factors which could explain why some parts of my image are near perfectly rectified (the LiDAR and resampled image match) and others are slightly offset?
Please let me know if I need to clarify any of these questions.