Intro to Spatial Analysis (mostly 2D)

CS 128/ES 228 - Lecture 12a 1

Intro to Spatial Analysis (mostly 2D)


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?


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)


Time for some geometry!


“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!)


Conventional Distance The Pythagorean

Theorem helps us compute “conventional” distances in the plane

Of course ArcMap does it automatically


“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?


Not your mother’s “Distance”

More complex distances require more complex analysis


Area (by vector) Area of a

rectilinearly aligned trapezoid is easy. A B

CC*(A+B)/2


Area (by vector)

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


Area (by Raster) Simply count the

rasters inside the polygon

orHow big is this?


Area (by ArcMap)


Points in Polygon Send out a

“ray” and count the crossings. ODD implies

inside EVEN implies

outside3 Crossings => INSIDE2 Crossings => OUTSIDE


Overlaying vector layers

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


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?


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


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?


DIGRESSION: What are rasters?

Vector layers with a single attribute datum?


Overlaying Rasters Simple Mathematics will often

suffice

But there is less information


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


A Sample ReclassificationLand Use

Old value

New value

“Other” new value

Wetland 7 1 4Road 10 0 0Lake 12 1 7Forest 14 0 1


Buffering – another tool Buffering (building

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


Using Buffers to Select

• Select the features

• Save the features as a layer

• (Export)


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: Box 6.5, pp. 187-88


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)


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.

Documents

Intro to Spatial Analysis (mostly 2D)