spatial analysis with arcview: 2-d

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Spatial Analysis with ArcView: 2-D

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Spatial Analysis with ArcView: 2-D. Calculating viewshed Calculating line of sight Add x and y coordinates Deriving slope from surface data Deriving aspect from surface data deriving flow direction calculate flow length delineating watershed. Visibility Analysis. Two types - PowerPoint PPT Presentation

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Page 1: Spatial Analysis with ArcView: 2-D

Spatial Analysis with ArcView:

2-D

Page 2: Spatial Analysis with ArcView: 2-D

– Calculating viewshed

– Calculating line of sight

– Add x and y coordinates

– Deriving slope from surface data

– Deriving aspect from surface data

– deriving flow direction

– calculate flow length

– delineating watershed

Page 3: Spatial Analysis with ArcView: 2-D

Visibility Analysis

• Two types– (1) Viewshed analysis identifies the areas on

a surface that are visible from one or more observation points. It answers the question, 'What can I see from these locations?'

– (2) Line of sight tells you whether a given target is visible from a particular point of observation. It answers the question, 'Can I see that from here?'

Page 4: Spatial Analysis with ArcView: 2-D

ArcView Spatial Analyst Calculate Viewshed

Surface/calculate viewshed

Page 5: Spatial Analysis with ArcView: 2-D

Viewshed Fields

• SPOT

• OFFSET• RADIUS

Page 6: Spatial Analysis with ArcView: 2-D

SPOT

• Defines the height of the observation point.– The SPOT field is useful when all

observation points are located at absolute heights.

Page 7: Spatial Analysis with ArcView: 2-D

SPOT

• A SPOT field has been added to the observation theme and given values of 3000 meters for each point. (The highest elevation in the grid theme is 1,966 meters.) These values might represent visibility from an airplane at each of the five observation points.

Page 8: Spatial Analysis with ArcView: 2-D

OFFSET

• The viewshed has been calculated using an OFFSET field with values of thirty meters above each observation point.

Page 9: Spatial Analysis with ArcView: 2-D

RADIUS

• Limit the visible distance from each observation point.

Page 10: Spatial Analysis with ArcView: 2-D

RADIUS

• Here you see a visibility grid created with RADIUS values of 1,000 meters

Page 11: Spatial Analysis with ArcView: 2-D

Line of sight analysis

• Determines whether a given target is visible from a given observer's point of view.

Page 12: Spatial Analysis with ArcView: 2-D

Line of sight analysis

A line of sight was drawn from the lower left to the upper right over the mountain.

Green line segments indicate visible terrain along the line graphic.

Red line segments indicate terrain that is not visible.

Page 13: Spatial Analysis with ArcView: 2-D

Line of sight visibility profile for the line drawn across Mt. Ranier.You can see the profile of the mountain. Green line segments represent visible terrain. Red line segments represent hidden terrain.

Page 14: Spatial Analysis with ArcView: 2-D

Deriving Slope and Aspect

• Slope identifies the maximum rate of change in values between each cell and its neighbors.

• Aspect identifies the down slope direction of the maximum rate of change in value from each cell to its neighbors.

Page 15: Spatial Analysis with ArcView: 2-D

Deriving Slope

• The output slope grid contains values representing degree of slope (from 0-90).

• Slope values can also expressed as a percentage.

Page 16: Spatial Analysis with ArcView: 2-D

Slope

Page 17: Spatial Analysis with ArcView: 2-D

e1 e2 e3

e4 C0 e5

e6 e7 e8

Horn’s algorithm for computing slope and aspect at C0

using the eight neighboring cells of C0

(((e1+2e4+e6)-(e3+2e5+e8))2+((e6+2e7+e8)-(e1+2e2+e3))2)0.5

S= ____________________________________________________________________________

8d

Page 18: Spatial Analysis with ArcView: 2-D

Slope theme of Mt. Ranier. Areas with darker shading have a steeper slope than those with lighter shading.

Page 19: Spatial Analysis with ArcView: 2-D

e1 e2 e3

e4 C0 e5

e6 e7 e8

Slope aspect can be computed by

D=arctan(((e6+2e7+e8)-(e1+2e2+e3))/((e1+2e4+e6)-(e3+2e5+e8)))

Page 20: Spatial Analysis with ArcView: 2-D

Aspect Directions

A cell with an aspect value of 90 is angled or facing east.If you walked down that hill, you would be walking east.

Page 21: Spatial Analysis with ArcView: 2-D

Make sure the same unit of x,y and z values, if not:

Using Map calculator to convert DEM by multiplying or dividing a factor 3.281 .

Page 22: Spatial Analysis with ArcView: 2-D

Add x and y to attribute table using the Field Calculator

• Add a theme and open the attribute table• theme/Start Editing/Edit/Add Field/x field• double click the new field and then go to

Field/Calculate menu.• The field calculator will appear and will ask

for a formula for the x-coordinate. Type .GetX. Then click ok.

• Same way to extract y values

Page 23: Spatial Analysis with ArcView: 2-D

Calculate flow direction

Page 24: Spatial Analysis with ArcView: 2-D

67 56 49

52 48 37

58 55 22

30

67 56 49

52 48 37

58 55 22

30

45.0230

4867

50.0

30

5267

Slope:

Direction of Steepest Descent

Page 25: Spatial Analysis with ArcView: 2-D

4

5

6

3

7

2

1

8

Eight Direction Pour Point Model D8

Page 26: Spatial Analysis with ArcView: 2-D

• Generating Surface Parameters - Flow Direction– If a cell is lower than its eight neighbors, that

cell is given the value of its lowest neighbor and flow is defined towards this cell.

– If a cell has the same slope in multiple directions and is not part of a sink the flow direction is calculated by summing the multiple directions

Page 27: Spatial Analysis with ArcView: 2-D

Grid Network

Page 28: Spatial Analysis with ArcView: 2-D

• Generating Surface Parameters - Flow Accumulation– Accumulated flow is calculated as the

accumulated number of all cells flowing into each downslope cell.

Page 29: Spatial Analysis with ArcView: 2-D

– For an accumulation surface the value of each cell represents the total number of cells that flow into an individual cell

– Cells that have high accumulation are areas of concentrated flow and may be used to identify stream channels.

Page 30: Spatial Analysis with ArcView: 2-D

1 1 111

1

1

1

1

1

1

1

1

14 3 3

12 2

2 163 6

25 2

1 1 11 1

1

1

1

1

1

1

1

1

1

4 3 3

12 1

2

23

16

256

Contributing Area Grid

Page 31: Spatial Analysis with ArcView: 2-D

1 1 11 1

1

1

1

1

1

1

1

1

1

4 3 3

12 2

2

23

16

256

Contributing Area > 10 Cell Threshold

Page 32: Spatial Analysis with ArcView: 2-D

Watershed Draining to This Outlet

Page 33: Spatial Analysis with ArcView: 2-D

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

Page 34: Spatial Analysis with ArcView: 2-D

• Generating Surface Parameters - Flow Length– Another parameter that can be calculated is

flow length– Flow length is defined as the length of the

longest flow path within a drainage basin or watershed.

Page 35: Spatial Analysis with ArcView: 2-D

2 2 2 1 128

1 1 1 12864

3264128128128

128 64 100 32 80

Flow Direction Surface

144.6 114.6 84.6 30 0

132.3 102.3 72.3 42.330

84.672.384.6114.6144.6

156.9 126.9 114.6 126.9 0

Flow Length Surface

Assuming 30 meter sq. cells

Page 36: Spatial Analysis with ArcView: 2-D

• Generating Watersheds and Catchments– Now you are ready to start delineating

watershed basins– Watersheds or Catchment areas are the basis for

many hydrologic analysis– Watersheds can be created for any spot in a

dataset.

Page 37: Spatial Analysis with ArcView: 2-D

Pour Point Contributing Area

Page 38: Spatial Analysis with ArcView: 2-D

Drainage Density for Different Support Area Thresholds100 grid cell threshold 1000 grid cell threshold

Page 39: Spatial Analysis with ArcView: 2-D

Suggestion:

One contributing area threshold does not fit all watersheds.

Page 40: Spatial Analysis with ArcView: 2-D

• Wetness Index– wetness index = ln(As /

tanB), where:• As = Contributing Catchment

Area in meters squared

• B = Slope of cell measured in degrees

Page 41: Spatial Analysis with ArcView: 2-D

• The Whole Process - a Flowchart

Page 42: Spatial Analysis with ArcView: 2-D

Are there any questions ?

AREA 1AREA 1

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