Topographic Maps vs DEM Topographic Map 1 24

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 720 Contours 740 720 700 680

DEM Elevations Contours 700 680

Digital Elevation Model 67 56 49 46 50 53 44 37 38 48 58 55 22 31 24 61 47 21 16 19 53 34 12 11 12

Cell Definition 30 m cell 67 56 49 46 50 53 44 37 38 48 58 55 22 31 24 61 47 21 16 19 50 (cell value) 53 34 12 11 12 cell

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

“Burning In” the Streams - Improve accuracy based on photograph 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 + =

The Flow Direction Grid

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

Direction of Steepest Descent 1 Slope: 1 67 56 49 53 44 37 58 55 22

Flow Direction Grid 2 2 4 4 8 1 2 4 8 4 128 1 2 4 8 2 1 4 4 4 1 1 1 2 16

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

Grid Network

The Flow Accumulation Grid

Flow Accumulation Grid 0 0 3 0 0 0 Grid Network 0 2 11 1 2 5 0 0 2 0 0 15 24 1 0 1 Count the # of contributing cells

Flow Accumulation Grid 0 0 0 3 2 2 0 0 0 11 0 0 1 15 0 0 2 5 24 1

Defining the Stream Grid

Flow Accumulation > 5 Cell Threshold 0 0 0 3 2 2 0 0 0 11 0 0 1 15 0 0 2 5 24 1

Stream Network for 5 cell Threshold Drainage Area 0 0 3 0 0 0 2 0 0 0 11 1 0 2 1 0 15 5 24 1

Streams with 200 cell Threshold (>18 hectares or 13. 5 acres drainage area)

1000 Cell Threshold Exceeded at Stream Junction 510 989 1504 (>1000)

Defining Stream Segments

Stream Segments 0 0 3 0 0 0 2 0 0 0 11 1 0 2 1 0 15 5 24 1

Stream Segments in a Cell Network 1 1 1 2 3 4 4 4 2 3 4 4 3 5 5 6 6 6 5 5

Creating Subwatersheds

Watershed Outlet

Watershed Draining to This Outlet

Watershed and Drainage Paths Delineated from 30 m DEM Automated method is more consistent than hand delineation

Subwatersheds for Stream Segments Same Cell Value

Vectorized Streams Linked Using Grid Code to Cell Equivalents Vector Streams Grid Streams

Delineated Subwatersheds and Stream Networks