Terrain and Watershed Analysis 1
Digital ElevaBon Model (DEM) raster of elevabons 2
Slope magnitude of gradient tan(slope) = ( ( z/ x) 2 + ( z/ y) 2 ) 1/2 3
Aspect (aka Exposure) direc,on of gradient tan(aspect) = - ( z/ y) / ( z/ x) NB: cyclical wraps around at 360 this image white: south black: north 4
Slope & Aspect: CalculaBon Neighborhoods 4 nearest neighbors 8 nearest neighbors e{2,4,5,7}: weight 2 e{1,3,6,8}: weight 1 5
Hillshade local sun angle cos(z) = cos(z sun ) cos(s) + sin(z sun ) sin(s) cos(a sun -A) this image Z sun = 47.5 A sun = 180 6
Hillshade with Shadows Shadowed pixels 0 7
Hillshade with Northwest IlluminaBon this image Z sun = 45 A sun = 315 Why NW? we re used to seeing things illuminated from above otherwise terrain looks reversed 8
Aspect vs Hillshade aspect hillshade Zsun = 47.5 Asun = 180 9
DEM ResoluBon 30m (USGS) 10m (USGS) 1.83m LIDAR 10
DEM ResoluBon: Derived Products Slope layers derived from three previous DEMs 11
DEM Rendering: Hypsometric Map Classified DEM color elevabon band 12
DEM Rendering: Contour Map Lines of constant elevabon 13
Triangulated Irregular Network (TIN) non-overlapping triangles 14
DEM vs TIN DEM TIN 15
TIN Breaklines Force triangle edges to follow exisbng features roads, streams, etc. a + breakline b = c 16
Visibility: Sightlines Sightline = visibility between points white: visible black: obscured 17
Visibility: Viewshed Viewshed = area visible from point 18
Viewshed = f(elevabon) (b) higher than (a) larger viewshed 19
Viewshed = f(search Radius) (a) edge of DEM (b) fixed radius 20
Viewshed: CumulaBve green areas visible from at least one point pink areas invisible from any point 21
Viewshed: Specific Cell values code all possible combinabons of search points code = visible from points(code) 22
Solar RadiaBon and Terrain Direct from sun Diffuse from sky Reflected from terrain ArcGIS ignores this 23
Solar RadiaBon: Sources Sunmap direct solar radiabon from each sky direcbon Skymap diffuse solar radiabon from each sky direcbon 24
Local Solar RadiaBon: Sky Viewshed topography upward-looking hemispherical viewshed 25
Local (Point) Solar RadiaBon sky viewshed + direct sunmap direct radiabon sky viewshed + diffuse skymap diffuse radiabon 26
Areal Solar RadiaBon: Diffuse 27
Diffuse RadiaBon as f(ridges & Valleys) DEM Diffuse radiabon 28
Areal Solar RadiaBon: Direct 29
Direct RadiaBon as f(aspect) Aspect Direct radiabon 30
Areal Solar RadiaBon: Global (Direct + Diffuse) 31
Watershed Analysis Watershed area that drains surface water to a common outlet defined by topographic divides Watershed analysis use to DEMs raster operabons delineate watersheds derive topographic features e.g. stream networks 32
Watershed DelineaBon Area-based area drained by selected stream Point-based area that drains through selected point 33
Automated Watershed DelineaBon 1. Fill sinks (depressions) in DEM 2. Derive flow direcbon raster direcbon water will flow out of each cell 3. Derive flow accumulabon raster tabulate number of cells that flow into each cell 4. Derive stream network apply threshold value to flow accumulabon raster 5. Derive stream links assign unique value and flow direcbon to each link 6. Delineate watershed using flow direcbon raster stream link raster 34
Flow DirecBon For 8-neighborhood: calculate distance-weighted drop 4 immediate neighbors: center - neighbor 4 corner neighbors: (center - neighbor) / 2 flow direcbon = maximum drop 35
Flow AccumulaBon A. filled elevabon raster B. flow direcbon raster C. flow accumulabon raster shaded cells have same accumulabon value top cell receives flow from lev and lower-lev cells bowom cell receives flow from lower-lev cell already had accumulabon value of 1 36
Flow AccumulaBon darker symbols = higher accumulabon values 37
Stream Network flow accumulabon raster stream network (threshold = 500 cells) stream network (threshold = 100 cells) 38
Stream Network stream network from 1:24000 DLG stream network (threshold = 500 cells) stream network (threshold = 100 cells) 39
Stream Links Assign unique value to each stream network secbon 40
Watersheds unique stream link unique watershed 41
Point-Based Watersheds watershed = f(outlet) aka pour point e.g.: stream gage stabon dam drinking water system intake 42
Snap Pour Point If pour point not on stream link then small, incomplete watershed Snap Pour Point to highest flow accumulabon within specified distance 43
Deriving a Point-Based Watershed If pour point at juncbon then watershed = merge(upstream watersheds) Else if pour point between juncbons then split current watershed watershed = upstream porbon + merge(upstream watersheds) 44
ResoluBon Affects Watershed Analysis 30m DEM 10m DEM 45
ResoluBon Affects Watershed Analysis stream network from 30m DEM stream network from 10m DEM 46
Relief Affects Watershed Analysis gray raster lines = stream segments from D8 method thin black lines = stream segments from 1:24,000- scale DLG agreement: good in valleys poor in flat areas 47
Figure Credits IntroducBon to Geographic InformaBon Systems, 4th ed. ISBN 978-0-07-305115-2 ArcMap Help 48