GIS/GPS Chapter 9

Buffering

creation of a special-purpose polygon that’s a specified distance around a point, line, or area (polygon) feature, creates 2 geographic areas

What are the 2 geographic areas buffering creates?

areas within the specified buffer distance, area beyond the buffer limit

Series of buffers

interval 1 to 30 and at some point the buffers start to merge with one another

Example of series of buffers

roads, fire hydrants, any utilities, distance of store from houses around it

Linear buffers

streams, train tracks

What’s the difference between polygons merging in buffers?

with or without dissolve on

Polygon based buffering

difficulties and do they violate rules/mandates

Negative buffering

decreases the size of the original polygon

Buffer dimensions

nature of the spatial-analysis questions dictates the size of the buffer dimensions and whether or not they’re systematic

Types of buffering

arbitrary, causative, mandated

Arbitrary

based on GIS analyst’s best estimate/guess of what size of the buffer should be, decision isn’t based on scientific principles, political mandates, or laws, allow air on the side of caution, and over-estimate buffers

Example of arbitrary

buffer is 3 m, but just go to 4 m, precaution

Causative

when the landscape/conditions surrounding the feature are non-uniform (heterozygous) and the analyst knows of this a priori (before the fact) then you want to apply causative buffering logic to identify buffer distances

Example of causative

roads going through agricultural areas

Mandated

most straightforward, federal, state, local, and community government agenices routinely mandate rigorously defined buffer dimensions for specific types of features

Example of mandated

federal, state, local government mandates

Create thiessen polygon

making a line halfway 2 points, closest to the line within the polygon, splits up the area into polygons, use this for network and business analytics (areas physically closer to other stores)

Generate near table

calculates the distance in the features between 1+ layers, near tools modifies the input and puts it onto the file, just gives you the table not the file

Graphic buffer

adds fun shapes, are corners round or sharp

Multiple ring buffer

adds outer layers to the original polygon, each buffer is non-overlapping, can set at the same time

Near

what points are closest to what road, track, etc, 2D vs 3D distance is the elevation and makes the difference

Point distance

near tool but determines the distance from all the points, uses the search radius

Polygon neighbors

tells you what’s nearby, creates a table with the edge of neighbors and see the amount of overlap, useful for cadastral information (parcels)

Overlay

extremely important, comparison of 2+ thematic features (maps) can be done using vector (topological) overlay/using raster overlay logic, vector object it results in a new topologically structured vector feature layer that includes the geometric and attribute characteristics of each of the input feature layers based upon the overlay operations that are applied

Types of topological overlay

point-in-polygon, line-in-polygon, polygon-on-polygon

Point-in-polygon

overlay of a point layer on a polygon layer

Line-in-polygon

overlay of a line layer on a polygon layer

Polygon-on-polygon

overlay of a polygon later on a polygon later

Vector overlay operations

intersection, union, symmetrical difference, identity, clipping, erasing, splitting

Intersection

based on use of Boolean and connector that computes the geometric intersection of the input layer and base feature layer

Union

based on the use of Boolean/connect and preserves all the features in both the polygon and base layer, preservation of polygon area for anything over the overlap

Symmetrical difference

use XOR connector when one wants to detect differences between layers, computes geometric intersection of the input layer features and the base layer features

Identity

computes geometric intersection of the input layer features and identify layer features, input features, or portions that overlap the identity layer, get its attributes

Clipping

extracts a geographic piece of the input layer using the base layer as a cookie cutter template, no attributes are combined

Erasing

deletes a part of the input layer using the base layer as a template for the geographic area to be deleted, opposite of clipping

Splitting

divides the input layer into a number of small layers based on the characteristics of the base layer, good for map projection problems

Overlay analysis example

lake murray dam, find an area with trees and see red it’s a false color projection, use any of the vector overlap operations, government and military bases would want to erase maps of property using the erase tool, union of all to be left with a smaller amount of certain area

Select

extracts features from an input feature class/input feature layer, typically using a select/structured query language (SQL) expression and stores them in an output feature class

Example of select

massive soils dataset but only want burrow picks and can pick only those areas and put it into its own file

Split by attributes

split and input dataset by unique attributes, input table, target workspace, and split field needed

Table select

select table records matching a structured query language (SQL) expression and writes them to an output table

Update

parcels of land (used the most here) and gets updated with the new data, frequency, summary statistics, tabulate intersection

Frequency

reads a table and a set of fields and creates a new table containing unique field values and. the number of occurrences of each unique field value

Summary statistics

calculates a variety of simple statistics in a table

Tabulate intersection

computes the intersection between 2 feature classes and cross-tabulates the area, length, or count of the intersecting features

What does a split by attribute table need?

input table, target workspace, split field