topographic maps vs dem. topographic map 1:24,000 scale 20 ft contour 100 ft contour stream center...
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Topographic Maps vs DEM
Topographic Map1:24,000 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 MeshStandard 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
720 720
DEM Elevations Contours
700
680
67 56 49 46 50
12 11 12
53 44 37 38 48
58 55 22 31 24
61 47 21 16 19
3453
Digital Elevation Model
67 56 49 46 50
12 11 12
53 44 37 38 48
58 55 22 31 24
61 47 21 16 19
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
67 56 49
53 44 37
58 55 22
1
67 56 49
53 44 37
58 55 22
1
26.162
4467=
−14
1
5367=
−Slope:
Direction of Steepest Descent
2 2 4 4 8
1 2 16
1 2 4 8 4
128 1 2 4 8
2 1 4 4 4
11
Flow Direction Grid
Flow Direction Grid- Red is NE
32
16
8
64
4
128
1
2
Grid Network
The Flow Accumulation Grid
0 0 000
0
0
0
0
0
0
0
0
03 2 2
11 1
1 152 5
24 1
Flow Accumulation Grid
Grid Network Count the # of contributing cells
0 0 00 0
0
0
0
0
0
0
0
0
0
3 2 2
11 1
1
12
15
245
Flow Accumulation Grid
Defining the Stream Grid
0 0 00 0
0
0
0
0
0
0
0
0
0
3 2 2
11 1
1
12
15
245
Flow Accumulation > 5 Cell Threshold
Stream Network for 5 cell Threshold Drainage Area
0 0 000
0
0
0
0
0
0
0
0
03 2 2
111
115
2 5
241
Streams with 200 cell Threshold(>18 hectares or 13.5 acres drainage area)
9895101504 (>1000)
1000 Cell Threshold Exceeded at Stream Junction
Defining Stream Segments
0 0 000
0
0
0
0
0
0
0
0
03 2 2
111
115
2 5
241
Stream Segments
55
11
1
3
22
334 4 4
4 45
5
666
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
VectorStreams
GridStreams
Delineated Subwatersheds and Stream Networks