# Somewhat less 8-bit hachures

The previous version was biased toward diagonals because it didn't take into account that diagonal movement from each point is a longer distance than moving straight up, down, left, or right, and that therefore it shouldn't move that way unless the drop is sqrt(2) as long in that direction. This also comes out a little darker because it is only invalidating a circle around each point instead of a square.

probable koz, Gem Images, and Ross added this photo to their favorites.

1. The confused fuzz on flat land is particularly interesting. Are your particles traveling uphill or down?

2. The main part of the San Andreas fault stands out so distinctly in these images. These are great. Thanks for sharing them.

3. They're traveling downhill, but there probably ought to be a threshold that keeps them from drawing if they only go a step or two. I should let them go uphill too and see what that looks like.

4. It looks pretty much the same if you let them go uphill instead. I think there's just a lot of noise in the elevations.

5. Is this SRTM 30m? It has some terrible data in flat areas, just awful. I had to skip it entirely in DEM tools in favor of a crossfade between SRTM 90m and NED 10m.

6. This is the SRTM1 (1 arc-second/30 meter) data from dds.cr.usgs.gov/srtm/version2_1/SRTM1/ I think what's going on in flat areas is that it can't tell the difference between buildings and land so you get whatever random thing the radar bounced off of (or nothing when it failed to bounce off of shallow water). Are there even worse problems in other areas?

7. By the way, the algorithm is:

Make a list of the coordinates of all points and sort it by elevation
For each point, starting from the highest elevation:
; If it is still valid, plot that point and invalidate it
; While there are any valid neighbors:
; ; Choose the neighbor with the steepest drop from this one
; ; Plot that point and invalidate it
; ; Set that point as the current point to choose the next neighbor from
; For each point along the path just plotted:
; ; Invalidate a radius around the point equal to the square root of its distance along the path

Indenting with semicolons since that was trendy last week.

8. Try NED 10m, the files are large but when downsampled the quality is great.

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