Topic 10: Vector Analysis

Question 1

Recognize some of the options and strategies for three major categories of GIS analysis operations: queries, measurement, and transformations

Answer 1

Queries: asking a question and having the dataset answer, does not change the data set

• spatial: based on a location
• spatial: interrogate the attribute tables through query language

Measurements: determining distance

• Pythagore method, flat surface
• great-circle distance, although earth is not a perfect circle
• manhattan calculates distances using the edge of raster cells

transformations
- buffers and overlay and point-in-polygon

Question 2

Perform some simple overlay and buffering operations

Answer 2

overlay:

• intersect: computes the intersection of the two layers
• clip: takes out point of interest
• erase: erases where the feature layer is placed
• union: combining the two layers
• identity: two layers intersect, input layer will get the identities of the identity feature where they overlap

buffer:

• builds object around existing point, line, or polygon
• can be done with both raster and vector

Question 3

Processing in Raster & Vector

Answer 3

• Beginning geospatial technicians often think of raster and vector data analysis as two different worlds
• In reality, most software packages provide relatively seamless integration between the two (Spatial Analyst in ArcGIS Pro)
• Which is better?
  Actually, most operations can be accomplished with both data models
  Conversion between the two can introduce errors
  Often best to stick with the “native” data format

Question 4

Entity

Answer 4

An individual point, line, or polygon in a GIS database

Question 5

Attribute

Answer 5

Data about the entity

• In a vector GIS, attributes are stored in a database, and there may be many attributes for given entity
• In a raster GIS, the grid code represents the attribute for the pixel, and there typically is only one attribute per entity

Question 6

Data layer/coverage

Answer 6

one of a series of themes or data sets in a GIS, consisting of entities and their associated attributes
- Note: the general term ‘coverage’, as I’ve used here, is actually a bastardization of the original term, which refers to the Arc/info native file format. However, it’s used so often in this context (data layer) that I’m including it

Question 7

Image

Answer 7

a raster data layer
- Note that images (rasters) are not limited to remote sensing imagery. Anything that we store ‘pixel by pixel’ is a raster

Question 8

function/ operation

Answer 8

a data analysis procedure performed by a GIS

Question 9

Algorithm

Answer 9

an unambiguous sequence of functions/ operations designed to solve a problem

Question 10

Queries

Answer 10

Questions posed to the database that do not involve changes to the database

Question 11

What is the difference between spatial and aspatial?

Answer 11

Spatial queries operate “by location” (found under the selection menu in ArcGIS Pro)

Aspatial queries interrogate the attribute tables through structured query language (SQL)

Question 12

What are the different Boolean expressions?

Answer 12

Boolean logic is based on the evaluation of binary values (yes/no, on/off, 1/10)

Complex queries can be created by combining individual expressions (spatial or aspatial) through boolean operators

• AND: true if both are true
• OR: true if either is true
• XOR: true if exactly one is true
• NOT: inversion expression

Question 13

Measurements

Answer 13

simple measurements regarding geographic entities or attributes
- The ability of computers to make accurate and consistent area measurements was the original motivations for the Canada Geographic Information System (CGIS): the world’s first GIS

Question 14

How to determine distance measurements using the simplest method?

Answer 14

simplest method of determining distance between points in space is the Pythagorean or Euclidean distance

• Distance between A and B defined by hypotenuse of a right-angle triangle
• Assumption is that the points lie on a flat plane, and, of course, the world is not flat!

Question 15

What is the great circle distance?

Answer 15

Great circle distance calculates the metric for a spherical earth

Again, though, the earth is not completely spherical (geoid), so even more complex procedures may be called for under certain circumstances

Question 16

What is Manhattan distance?

Answer 16

Manhattan distance calculates the distance along raster cell sides

Again could result in a distortion created by the data model

Question 17

Area and perimeter measurements

Answer 17

GIS perimeter estimates are a fairly straightforward extension of distance measurements

• Summing up the lengths of the polylines defining the polygon in a vector layer
• Multiplying the number of cell sides that make up the boundary of the feature by the pixel size

Question 18

Transformations

Answer 18

simple methods of spatial analysis that transform attributes or entities of geographic data into useful NEW products

Buffers, overlays, point-in-polygon, interpolation: these relatively simple operations form the basis of powerful algorithms that solve a multitude of geographic problems

Question 19

Buffers

Answer 19

A buffer operation builds NEW objects around existing point, line, or polygons by identifying all areas within a specified distance
- Among the most useful and popular GIS operations

Question 20

Buffers: Raster vs Vector

Answer 20

Buffering can be done with both raster and vector layers, but rasters enable the simple extension of buffering to consider factors other than simple distance of buffering to consider factors other than simple distance

the application on a raster is more flexible in some applications

Question 21

Point in polygon

Answer 21

the point-in-polygon operation determines wheter a point lies inside or outside a polygon, though it can be extended to consider many points and many polygons

Question 22

How does the algorithm determine if a point is in a polygon?

Answer 22

There is a line that extends for infinity, if it crosses the polygon an odd number of times it is in the polygon. If it crosses an even amount of times it is outside the polygon

Question 23

Polygon overlay

Answer 23

involve the spatial intersection of two different polygon layers, and are among the most varied and powerful families of GIS operations

Question 24

overlay operations: identity

Answer 24

• Take cookie cutter, plop it over top, creating breaks in the underlying data
• Make a break saying that segment contains attribute of cookie cutter
• Adds attribute

Question 25

overlay operations: intersect

Answer 25

• computes spatial intersection of the two layers
• where they are the same (Boolean logic)
• clips and combines

Question 26

overlay operations: union

Answer 26

both of them combined into one feature layer

Question 27

overlay operations: clip

Answer 27

taking out area of interest

Question 28

overlay operations: erase

Answer 28

(opposite of clip) erasees where the feature layer is placed

Question 29

Polygon overlay issues

Answer 29

Polygon overlay operations with vector datasets are problematic because of the potential for slivers being generated by the procedure

The same boundary (eg., coastline) may be represented slightly different in each layer, because of the slight digitizing errors
- Paradoxically, the more detail you put into the digitization, the more pronounced this problem becomes

Most software packages try to minimize this problem though tolerance thresholds, but these must be set correctly
- Not an issue in raster overlays, since the entities (pixels) are all the same

Question 30

Transformations: Spatial Interpolation

Answer 30

Spatial interpolation: algorithms that attempt to estimate values of a continuous field in places where no measurements exist in the database

• estimating rainfall in places away from individual weather stations
• estimating elevation from contour lines

Inverse distance weighted function

• measure straight line distance between points, distance weighted average
• one of the most common