Transcript No Slide Title

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
720
Contours
740
720
700
680
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
50
(cell value)
61 47 21 16 19
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
16
8
128
1
4
2
Direction of Steepest Descent
1
1
67
56
49
67
56
49
53
44
37
53
44
37
58
55
22
58
55
22
67  44
 16.26
Slope:
2
67  53
 14
1
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
0
0
0
Grid Network
0
2
0
0
2
0
0
11
1
2
15
5
24
1
0
1
Count the # of
contributing cells
Flow Accumulation Grid
0
0
0
0
0
0
3
2
2
0
0
0
11
0
1
0
0
1
15
0
0
2
5
24
1
Defining the Stream Grid
Flow Accumulation > 5 Cell Threshold
0
0
0
0
0
0
3
2
2
0
0
0
11
0
1
0
0
1
15
0
0
2
5
24
1
Stream Network for 5 cell
Threshold Drainage Area
0
0
3
0
0
0
0
0
2
0
0
2
0
0
0
11
1
0
1
0
15
2
5
1
24
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
0
0
2
0
0
2
0
0
0
11
1
0
1
0
15
2
5
1
24
Stream Segments in a Cell Network
1
1
2
1
2
3
5
3
3 5
4 4 4
4 4
6 6
6
5
5
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
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