rubber rulers

Dynamically Scaled Buffers and Distance — J.K. Berry calls ’em “rubber rulers”

Vector thinking…..What if the size of a buffer is not everywhere the same?  Perhaps because the underlying reason for the buffer varies. The width of a riparian forest buffer should depend on the size of the stream, larger buffers for larger streams if the goal is to mitigate flooding.  Here we create a variable called “buffer” in the river attribute table that is used to “scale” the buffer as it is applied to different segments. You can apply this dynamic buffer using the “Data Defined” dropdown menu next to Distance, and selecting the field from the attribute table you stored the dynamic buffer values for each stream. Where these buffers overlap, you can check the “Dissolve result” option.

QGIS attribute buffer size
Use the streams layer “buffer” field to control the buffer size
attribute controlled buffer distance
Note the use of the expression to control the line labels “title( “PNAME” ) || ‘ ‘ || “BUFFER” || ‘ m'”

In 2020, Virginia implemented a new GIS based system to identify high priority conservation land, based on several conservation goals (https://www.dcr.virginia.gov/conservevirginia/). For water quality, they first sought to “ identify watersheds (12-digit hydrologic units) with the highest (i.e., those in the 90th percentile) loadings of nitrogen, phosphorous, or sediment from any of the assessments used.” And then created stream buffers, dynamically scaled to slope. “Buffers were mapped for these waterways, where buffers ranged from 100 to 400 ft., depending on steepness of slope of adjacent lands. ” source  How’d the do that?? We can explore these dynamic buffers on Virginia’s Natural Heritage Data Explorer
map of stream buffers from VA natural heritage

Grid thinking…….We can also do buffer size based on land use.  Here I have…
1. simplified a land use map to 3 categories (land_use)
2. made a distance to stream map (dist2stream), then
3. used these two maps to select areas within given distances from the stream depending on the land use (Raster Calculator)

dynamic stream buffer raster
Use the raster calculator to run this statement (this or “CASE WHEN THEN)
(( "land_use@1" = 1) AND ( "dist_2_stream@1" < 400)) OR (( "land_use@1" = 2) AND ( "dist_2_stream@1" < 200)) OR (( "land_use@1" = 4) AND ( "dist_2_stream@1" < 100))

Further thinking….

Q: How about in vector world? How would you do the same thing