CS 128/ES 228 - Lecture 12a1 Intro to Spatial Analysis (mostly 2D)

CS 128/ES 228 - Lecture 12a 1

Intro to Spatial Analysis (mostly 2D)

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Some GIS Queries How big is the lake? What is the longest trail? How many fire hydrants on campus? Which dorms are within 100 m of an

academic building? Where is the best place for a new

dorm?

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Types of queries Aspatial – make no reference to

spatial data Which dorm has the highest occupancy

rate? (we can already do) Spatial – make reference to spatial

(and possibly attribute) data Which fire hydrant is closest to the

chemistry labs? (we can sort of do)

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Time for some geometry!

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“Simple” spatial queries How long is this line?

“Tricky” if line is a bunch of line segments

“Tricky” if distance isn’t Pythagorean How much area does this polygon

cover? (Can we do this?) Is this point in this polygon? (Can’t

do this!)

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Conventional Distance The Pythagorean

Theorem helps us compute “conventional” distances in the plane

Of course ArcMap does it automatically

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“Alternative” distance “Manhattan” distance How many blocks (via a

taxi cab) from A to B? ArcMap can do this in a

query/report

A

B

What about one-way streets?

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Not your mother’s “Distance”

More complex distances require more complex analysis

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Area (by vector) Area of a

rectilinearly aligned trapezoid is easy. A B

CC*(A+B)/2

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Area (by vector)

For a polygon, add up the (signed) trapezoidal areas

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Area (by Raster) Simply count the

rasters inside the polygon

orHow big is this?

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Area (by ArcMap)

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Points in Polygon

Send out a “ray” and count the crossings. ODD implies

inside EVEN implies

outside3 Crossings => INSIDE2 Crossings => OUTSIDE

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Overlaying vector layers

Spatial information (from layers) can be used to create new spatial information (i.e. new layers)

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Overlaying Layers (Intersection) Keep only those things that belong

to both layers

Example: Overlay my property with a hydrology layer Learn how much of my “land” is under

water. What to do about the property

boundary and the lake?

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Overlaying Layers (Intersection)

Keep any things that belong to either layer

Example: Overlay state highways layer and local roads layer to create pavement layer

Note: New Layer not actually created in this figure

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Overlaying Layers (Clipping)

Keep only those things from a given layer that lie within a specified boundary (often rectangular)

Example: Consider only those roads that lie within Cattaraugus County Problem: What if a road crosses the

boundary?

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DIGRESSION: What are rasters?

Vector layers with a single attribute datum?

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Overlaying Rasters Simple Mathematics will often suffice

But there is less information

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Effective Overlaying via Reclassification Data is not always in a good format

If raster pixels have different coverages, overlaying may be effectively impossible

Codings are generally categorical, not mathematical Adding codings usually does not make sense

Solution: RECLASSIFY

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A Sample Reclassification

Land Use

Old value

New value

“Other” new value

Wetland 7 1 4

Road 10 0 0

Lake 12 1 7

Forest 14 0 1

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Buffering – another tool Buffering (building

a neighborhood around a feature) is a common aid in GIS analysis

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Using Buffers to Select

•Select the features

•Save the features as a layer

•(Export)

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Putting it all together Siting a nuclear waste dump

Build Layer A by selecting good geology Build Layer B by reclassifying

population for high density Build Layer C by clipping B from A Build Layer D by buffering roads Build Layer E by intersecting C and D …

See also: Figure 6.9, p. 121

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Where does it fit in? GIS holds data Spatial analysis causes us to view

the data as information Combining queries turns that

information into knowledge

(It’s all a spectrum)

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Conclusions A GIS without spatial analysis is like

a car without a gas pedal.

There are some things you can still do with it, but it’s hardly worth maintaining the vehicle.