Continuous Field and Surface Analysis - Lecture Slides | CSS 4200, Study notes of Agricultural engineering

Download Continuous Field and Surface Analysis - Lecture Slides | CSS 4200 and more Study notes Agricultural engineering in PDF only on Docsity!

CSS 4200

Geographic Information Systems Lecture 12: – Continuous Fields– Surface Analysis**Bolstad, Chap 11

What is a Surface?

•^

Continuous entity

-^

Infinite number of points– Must use a subset of points in practice– Interpolate, at least to some level

-^

Often synonymous with elevation (terrain) surface

Surface Operations

-^

Profile

-^

Terrain derivatives– Slope gradient– Slope azimuth (deg), aspect (dir)– Slope curvature– Terrain ruggedness, surface roughness– Topographic indices

-^

Surface water flow, watersheds– Flow direction– Flow accumulation

-^

Line of sight, viewshed

-^

Illumination, shaded relief

-^

Cut/Fill volume

-^

“Fly-throughs,” drapes

Bilinear Interpolation

(distance weighted average)

Fig 4-31Bolstad

Slope Gradient

•^

Change in elevation– “rise over the run”– Calculated in percent, or degrees

-^

Generally calculated using a 3x3 moving window– 8 neighboring cell elevations subtracted from center

cell and divided by distance

• Use average or highest

Slope Gradient

•^

Other methods of calculation– 4 neighboring cells (rook’s case)– 3

rd

order finite difference

•^

Percent to degrees– tan

(% slope/100) = slope gradient in degrees

• radians to degrees conversion

Folded Aspect

•^

Used in calculating heat load

-^

Aspect is “folded” such that it runs from 0 to 180and back down

-^

Highest value set to the aspect that receives themost solar energy

-^

Northern hemisphere example:FA = 180° – ||aspect – 45°| – 180°|– Southwest = 180 °, Northeast = 0 °

Plan v. Profile Curvature

Slope Curvature

Concave (-) v. Convex (+)

Runoff and Watersheds

•^

Runoff measures downhill flow of water

-^

Watershed – the area of land that accumulatesand transmits precipitation input to a water body

-^

In theory, any water that falls in a givenwatershed will end up in that watershed’s waterbody (or evaporated or transpired…)

Watersheds

•^

Fill depressions (sinks)

-^

Calculate flow direction for each cell

-^

Calculate flow accumulation

-^

Delineate watershed

Flow Accumulation

-^

Flow accumulation can be usedto create a stream network

-^

The accumulated flow is basedupon the number of cells flowinginto each cell

-^

Output cells with a flowaccumulation of 0 are localtopographic highs:- Used to identify ridge lines, orwatershed divides.

-^

Assumes that all precipitation isrunoff, and there is nointerception, evapotranspiration,or loss to groundwater.