The ability to perform 'analysis' differentiates GIS from computer assisted mapping (CAD).
Both a GIS and a CAD can store data in layers but a GIS, rather than just displaying different layers, can perform analysis between them, both spatially and via attributes. (refer back to Lecture 1 )
1. Location query
yields attributes of that feature - a database function example attributes
2. Selection by attribute (e.g. where are all the pine stands)
Boolean Logic for queries (named after George Boole, a 19th century mathematician)
- Boolean function:
A type of expression based upon, or reducible to a true or false condition.
Examples of boolean logic using Boolean operators: (raster data)
||Greater than or equal to
||Less than or equal to
||Between the values of
||Contains the character string in the quotes
- Boolean Operators (to combine items in a table)
- XOR ( = either or, but not both)
These often require the use of brackets to avoid ambiguity in complex queries e.g.
type = pine AND age > 100 ... selects all old growth pine
type = pine OR type = fir AND age > 100.. selects any pine OR old growth fir
type = (pine OR type = fir) AND age > 100 .. selects old growth pine and fir
Maps A, B, and C represent the original map layers.
The shade areas in D, E, F, and G represent where a condition is met for those locations.
Map D shows :
"Where is condition A but not B met?"
e.g. Which areas have deep soils but NOT spruce?
Map E shows :
"Where are both conditions A and D met?"
e.g. Which areas have deep soils AND spruce?
Map F shows :
"Where are either conditions B or C met?"
e.g. Which areas have spruce OR good drainage?
Map G shows :
"Where is condition B or C, but not both, met?"
e.g. Which areas have either spruce OR good drainage but NOT both?
Note: to examine attributes from separate vector data layers, these must frst be overlain (Overlay below)
3. Types of Analysis in GIS
Commonly used analytical tools in arcgis : append, buffer, clip, dissolve, integrate, intersect, union, erase
a. Database Query
- area, perimeter, values
overlay: compare different layers and combines attributes: points lines polygons subdivision into new polygons
point and line in area overlay queries area - area overlay queries
union versus intersect union intersect choice of union/intersect
- modify by a given factor: add, subtract, multiply, divide
- modify by projection, datum or geocorrect, clip and erase
- dissolve, append, join, group, merge, generalize
- cost surfaces, distance to features, buffer buffer example
- a system of interconnected lines - e.g. hydrological, transportation, utilities, animal migration: network analyst
- filtering, smoothing, 3D surfacing
- e.g. fire spread: based on fuel type, wind speed, direction, buffers, topography
Change : change effects, e.g. polar ice; beetle spread fires, volcanoes etc..
Things you should know after finishing this lecture:
- How is a database containing attribute tables essential for most GIS analysis?
- Name four general types of analysis in GIS and give specific examples in two
- What is meant by a boolean function
- List the boolean operators
- Why should a 'OR' query yield a larger result than a similar 'AND' query ?
Source: Source: http://www.gis.unbc.ca/courses/geog300/lectures/lect2/index.php