LAND scape representaion

1. Digital Elevation model
Grid size and Landscape
representation.
Presented by :-Abhishek vats
Id no. :- 18mtaeswc012
Submitted to :- Dr. mohd. Aftab Alam

2. Introduction
 DEM model is used for characterization of local slope and
drainage area per unit contour length.
 Distribution of topographic attributes derived from a DEM
depends to some degree of grid size.
 In this paper we asses that how grid size affects topographic
representation and derive topographic attributes for two
catchment area using high resolution digital elevation data.
 we grid the same elevation data at several different scales to
isolate the effect of grid size on landscape representation.

3. Study area
(i) Mettman ridge near coos bay, oregon
(ii) Tennesse valley in marine county, california

4. Location map of study catchment

5. Mettman ridge
 Area :-0.3 km2
 Chanel head locations are controlled by shallow debris flow from small
unchandled valleys.
 Slope length :-30-50 m.
 Slope :- 300 -400 common,450 rare
 Scale of 1:4800 scale topographic base map derived.

6. Contour map of mettman ridge

7. Tennessee valley
 Area = 1.2km2
 Shallow land sliding dominates sediment transport in steep hollow and
side slopes.
 Slope lengths :-30-50m
 Less steep than mettman ridge
 Slope:-200 -300 common, excess of 400 are rare.

8. Contour map of Tennessee valley

9. Land scape representation
 SLOPE
 DRAINAGE AREA PER UNIT CONTOUR LENGTH
 TOPOGRAPHIC INDEX

10. Method
 Spot elevation data for two catchment ware gridded at scale of
2,4,10,30,90.
 Cumulative frequency of topographic attributes are calc. for each DEM
 The local slope for the cell is calc. based on the difference of elevation
between cells.
 By the help of no. of cells that directs the flow to each cells and thus
drainage area
 Topographic area is calc. by the formula
Topographic index = a/tan β
where a = drainage area per unit contour length
tan β= local slope

11. Slope
 As Cumulative frequency distribution of slope are more sensitive to
steeper slope
 Slope decreases as grid size increases
 For mettman ridge it declines from 0.65 for 2 m to .41for 90 m grid size
 For Tennessee valley it declines from .34 for 2 m to .29 for 90 m grid size

12. Cumulative frequency distribution of slope derived from different
DEM grid size

13. Drainage area per unit contour length
 Larger grid size – larger contributing area
 For mettman ridge it increases from 20m for 2 m grid size to 102m for 90
m grid size
 For tennesse it increases from 19m for 2 m grid size to 120m for 90 m
grid size

14. Cumulative frequency distribution of drainage area per
unit contour length derived from different DEM grid size

15. Topographic Index
 It reflects the soil moisture, surface saturation and and runoff generation
processes.
 Decrease grid size - lower value of topographic index
 For mettman ridge it increases from 3.4 for 2 m to 5.6 for 90 m grid size
 For Tennessee valley catchment it increases from 4.0for 2 m to 6.2 for 90
m grid size

16. Cumulative frequency distribution of topographic index
derived from different DEM grid size

17. Conclusion
 As grid size decreases landscape features are more accurately resolved.
 Representation of land surface by DEM depend on
(i) Grid size
(ii) Accuracy and distribution of original survey data from which the DEM
was constructed.

18. Result
A grid size of 10m would suffice for many DEM –based application of
geomorphic modelling.