# Topographic Maps vs DEM. Topographic Map 1:24,000 Scale 20 ft contour 100 ft contour Stream Center Line.

## Presentation on theme: "Topographic Maps vs DEM. Topographic Map 1:24,000 Scale 20 ft contour 100 ft contour Stream Center Line."— Presentation transcript:

Topographic Maps vs DEM

Topographic Map 1:24,000 Scale http://www.tnrcc.state.tx.us/gis/raster.html 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 720 700 680 740 680700720740 720

DEM Elevations Contours 700 680

6756494650 121112 5344373848 5855223124 6147211619 3453 Digital Elevation Model

6756494650 121112 5344373848 5855223124 6147211619 3453 50 30 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

32 16 8 64 4 128 1 2 Eight Direction Pour Point Model

675649 534437 585522 1 675649 534437 585522 1 Slope: Direction of Steepest Descent

22448 1216 12484 1281248 21444 11 Flow Direction Grid

Flow Direction Grid- Red is NE 32 16 8 64 4 128 1 2

Grid Network

The Flow Accumulation Grid

00000 0 0 0 0 0 0 0 0 032 2 11 1 1 15 25 24 1 Flow Accumulation Grid Grid Network Count the # of contributing cells

00 0 00 0 0 0 0 0 0 0 0 0 322 11 1 1 1 2 15 245 Flow Accumulation Grid

Defining the Stream Grid

00 0 00 0 0 0 0 0 0 0 0 0 322 11 1 1 1 2 15 245 Flow Accumulation > 5 Cell Threshold

Stream Network for 5 cell Threshold Drainage Area 00000 0 0 0 0 0 0 0 0 032 2 11 1 1 15 25 24 1

Streams with 200 cell Threshold (>18 hectares or 13.5 acres drainage area)

989510 1504 (>1000) 1000 Cell Threshold Exceeded at Stream Junction

Defining Stream Segments

00000 0 0 0 0 0 0 0 0 032 2 11 1 1 15 25 24 1 Stream Segments

5 5 1 1 1 3 2 2 3 3 4 4 4 4 4 5 5 6 6 6 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

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