Topographic Maps vs DEM
Topographic Map 1:24,000 Scale 20 ft contour 100 ft contour Stream Center Line
Flow in Direction of Steepest Descent
Watershed Delineation by Hand Digitizing Watershed divide Drainage direction Outlet
30 Meter Mesh Standard for 1:24,000 Scale Maps
30 Meter DEM Cell Boundaries
USGS 30 Meter DEM for This Area
DEM Lattice Points
DEM Cell Stores Elevation at Lattice Point
DEM Elevations Contours
DEM Elevations Contours
Digital Elevation Model
m cell cell (cell value) Cell Definition
Dealing with the Pits
Filling in the Pits DEM creation results in artificial pits in the landscape A pit is a set of one or more cells which has no downstream cells around it Unless these pits are filled they become sinks and isolate portions of the watershed Pit filling is first thing done with a DEM
+ = Take a mapped stream network and a DEM Make a grid of the streams Raise the off-stream DEM cells by an arbitrary elevation increment Produces "burned in" DEM streams = mapped streams “Burning In” the Streams - Improve accuracy based on photograph
The Flow Direction Grid
Eight Direction Pour Point Model
Slope: Direction of Steepest Descent
Flow Direction Grid
Flow Direction Grid- Red is NE
Grid Network
The Flow Accumulation Grid
Flow Accumulation Grid Grid Network Count the # of contributing cells
Flow Accumulation Grid
Defining the Stream Grid
Flow Accumulation > 5 Cell Threshold
Stream Network for 5 cell Threshold Drainage Area
Streams with 200 cell Threshold (>18 hectares or 13.5 acres drainage area)
(>1000) 1000 Cell Threshold Exceeded at Stream Junction
Defining Stream Segments
Stream Segments
Stream Segments in a Cell Network
Creating Subwatersheds
Watershed Outlet
Watershed Draining to This Outlet
Watershed and Drainage Paths Delineated from 30m DEM Automated method is more consistent than hand delineation
Same Cell Value Subwatersheds for Stream Segments
Vectorized Streams Linked Using Grid Code to Cell Equivalents Vector Streams Grid Streams
Delineated Subwatersheds and Stream Networks