Lecture 2

Question 1

4 Main Components of GIS

Answer 1

1.Hardware: the computer platform and peripherals associated with the GIS
▪ input devices: keyboard/mouse, scanner, digitizing tablet
▪ computer: high speed CPU and >6 GB RAM, and a large (or multiple) monitor are necessary for processing large spatial data sets
▪ output devices: high quality colour printers and large format plotters

2.Software: there are many different GIS programs out there, and you should compare your needs with the features associated with each program
▪ vector data requires a robust ability to process linear networks (eg, roads) and polygons (eg, land parcels)
▪ raster data requires a program that is adept at raster data analysis, and a computer able to handle large file sizes

3.Liveware: the characteristics and capabilities of the people responsible for designing, implementing, and using the GIS
▪ proper use of a GIS depends on people to structure the research questions, perform the analysis, and then judge whether the information extracted from the GIS is of value
-3 Main people involved-designer(ex:software engineers), technicians(ex: if something goes wrong), and User

4.Data: a successful GIS analysis needs specially prepared geospatial data
▪ garbage in, garbage out…

Question 2

if your only using Raster Data, ArcGIS is probably not the best- ______ becomes best

Answer 2

IDRISI

Question 3

What is an OGC and what is its function

Answer 3

Open Geospatial Consortium (OGC) is a global online community that works to make GIS interoperable by building links that allow the accurate and efficient transfer of information derived using many of the most popular commercial and public GIS Open Geospatial Consortium (OGC) is a global online community that works to make GIS interoperable by building links that allow the accurate and efficient transfer of information derived using many of the most popular commercial and public GIS

Question 4

Parts of the GIS business: 3

Answer 4

1.GIS software development
2.GIS data acquisition
3. GIS Value-Added Services
▪ a massive industry of businesses using GIS to answer geographic questions for clients

Question 5

in Canada, ________ is the primary portal for governmentbased spatial data

Answer 5

GeoGratis

Question 6

Town of Oakville example

Answer 6

-example of open sourced GIS which is available through city website

Question 7

_____-Parent company of ARCGIS

Answer 7

ESRI

Question 8

in reality, we typically identify two forms of spatial data:

Answer 8

1. Discrete
   - occur when data points can be located in a specific location and/or their values are either finite or a countable number of possible values
2. Continuous
   - occur when a phenomenon of interest continuously changes in space and/or contains values from infinitely many possible values

Question 9

Vector Spatial Model

Answer 9

the vector spatial model is what you think of when you draw a map
– they are made up of 3 basic features:
▪ polygons: 3 or more points connected by a closed set of lines
▪ lines: an x-y coordinate at each end
▪ points: an x-y coordinate pair

Question 10

Points in vector data

Answer 10

a point is the most basic vector feature, and are typically used to identify discrete features on Earth’s surface (oil wells, historic landmarks, telephone poles, fire stations, etc.)
▪ points do not have any spatial scale, so we cannot identify their size or shape, just their location
▪ points can have any number of attributes attached to them (age of the mine, depth of the main shaft, quality of the ore, tonnes extracted per month, number of employees, etc.)

Question 11

Lines in Vector Data

Answer 11

a line is a 1-dimensional feature that has location properties and length
▪ all lines have at least 2 points (beginning and end), often referred to as nodes
▪ other points may be located along the line, indicated where the line changes direction, and are called vertices

Question 12

Polygon in Vector Data

Answer 12

▪ a polygon is a 2-dimensional feature that is bounded by a continuous line, and has both size and perimeter properties
▪ polygons are composed of a start-end node and several vertices which define the shape
▪ the length of the perimeter line and the area of the space contained within the perimeter are fundamental properties of the polygon
▪ adjacent polygons can share nodes along common boundaries

Question 13

the vector model is strongly impacted by the _______

Answer 13

scale

▪ on a world map, a city is represented by a point, but as a polygon on a large scale map

Question 14

Topology

Answer 14

▪ topology is an important component of the vector model, and defines the relationship between features and describes how spatial data share geometry
▪ when using GIS, you will rarely work with a single spatial data set
– multiple data sets, especially if they come from multiple sources, will be slightly different
▪ for instance, consider 2 maps of Canada drawn at different scales
– GIS can plot both together but it is unlikely that the provincial borders fall exactly on one another
– this is a topological error
▪ consider topology to be a form of error correction
– all spatial data contain some form of error and topological correction seeks to eliminate these

Question 15

Name 2 types of topological error associated with lines

Answer 15

▪ open polygons occur when the polygon does not close, therefore it is a line feature, not a polygon
▪ overshoots and undershoots occur where line features that intersect in reality do not do so in the GIS
-overshoots go well past the line
-undershoots do not touch the line

Question 16

topology is based on 3 basic elements:

Answer 16

1. ▪ adjacency: information about the neighbourhoods of different objects
2. ▪ connectivity: information about the links between spatial objects
   - critical for road networking
3. ▪ enclosure: information about spatial features that enclose other spatial features

▪ these are important in spatial analysis
- eg, enclosure can be used to determine the presence of an island within a lake, or adjacency can be used to compare one polygon to those around it

Question 17

a georelational data model..

Answer 17

stores spatial and attribute data in separate files– spatial data are stored in graphics files and attribute data are stored in a relational database
▪ an identifier is used to link the
▪ unfortunately, this data model requires the software to constantly go back and forth between the spatial and attribute data
– this is slow and inefficient, but many older datasets still use this format

Question 18

What is a “Coverage”

Answer 18

▪ in GIS, a coverage is a digital vector storage framework with built-in topology
▪ a coverage is a framework– a collection of files and directories that are linked together to create a cohesive vector layer
▪ coverages are not single files that can be copied and pasted like a typical file – they must be maintained within the confines of special file management software built into the GIS
▪ separate tables are used for points, lines, and polygons and connected to the spatial data using an identifier

Question 19

T OR F

For Coverage, separate tables are used for points, lines, and polygons and connected to the spatial data using an identifier

Answer 19

T

Question 20

Shapefile

Answer 20

▪ shapefiles contain non-topological vector data and attribute information in a dataset – the geometry of features is stored as shapes that comprise a set of vector coordinates
▪ since shapefiles do not have the topological information like coverages, they do not have the same processing requirements and are generally able to read, display, and analyze more quickly and take up less disk space

-all shapefiles have at least 3 associated files: *.shp that holds the geometric shape; *.shx that contains an index file that links the geometry to the attributes; and *.dbf that contains the attributes

Question 21

Generalizations

Answer 21

a collection of objects with the same attributes, which can be grouped into superclasses of subclasses

Question 22

Geodatabase

Answer 22

an object-based model that maintains topologically integrated spatial datasets – vector data is stored as points, lines and polygons and further organized into feature classes and feature datasets

Question 23

3 kinds of geodatabase

Answer 23

1.file geodatabase:
stored as folders in a file system; ideal for single users or small workgroups

2.Personal Geodatabase:
stored in a MS Access data file; best used when there is a single data author/editor and a small number of users

3.ArcSDE geodatabase:
stored using ORACLE, MS SQL Server, IBM DB2, or IBM Informix; they are unlimited in size and number of users

Question 24

Raster Model

Answer 24

raster data have a more simple data structure than vector data and are represented in space by an array (or grid) of cells

cells are arranged in rows and columns, and each cell is usually square –The cells are commonly called picture elements or pixels(slide 29)

each cell contains a value that describes the phenomenon being examined (land use, vegetation type, elevation, brightness, presence/absence, etc.)

▪ raster data are especially useful for describing spatial phenomena that vary continuously across space
▪ air temperature is difficult to display as a vector – you can show individual measurement stations as points, but what about the space between stations?
▪ elevation is commonly represented as a special raster format known as a digital elevation matrix (DEM)

Question 25

What is DEM?

Answer 25

elevation is commonly represented as a special raster format known as a digital elevation matrix (DEM)
-floating point rasters

Question 26

whereas ______ data may contain a large number of attributes for each feature, _____ data contains one value for each cell

Answer 26

vector

raster

Question 27

Integer Rasters

Answer 27

typically represent nominal-scale categorical data such as land use or land cover type
▪ eg, a cell value of 1 may represent urban or built-up land, 2 may represent agriculture land, etc.
-whole numbers

Question 28

floating point rasters

Answer 28

floating point rasters are typically used for continuous data, where the cell value represents an actual data value
▪ eg, in a temperature raster, a cell value of 27.2 would represent an average air temperature of 27.2°C within that cell
–floating point raster(ex:DEM) require more space and computing power and memory to store query, and analyze

Question 29

What does cell size in raster model determine?

Answer 29

the cell size is important because a raster layer has just one value associated with each pixel – that value represents an average of that phenomenon in that pixel ▪ if the pixel were bigger, there would be more averaging and you would lose fine details; if the pixel were smaller, you would highlight tiny details that may not be important
▪ in addition, the file size of a raster is dependent on the total number of pixels – a lot of small pixels results in a large file size, while a few large pixels would yield a small file size  finding the right balance between information contained in the raster and its file size is important

a cell size of 30 x 30 m encompasses a geographic area of 900 m2, whereas a 5 x 5 m cell encompasses and area of 25 m2. The 5 x 5 m cell size is a higher resolution than the 30 x 30 m size

Question 30

Which is higher resolution and why?

5m x 5m or 30m x 30m

Answer 30

5 x 5-more detail in smaller area

30 x 30 looks way better though, and is more accurate

Question 31

Area covered by resolutions:

5m x 5m=

30m x 30m=

Answer 31

25 m2

900m2

Question 32

vector model uses a ________-type reference system

Answer 32

Cartesian

Question 33

spatial reference system for raster data sets?

Answer 33

raster coordinates are based on defining the spatial coordinates of the upper-left cell, and then relying on the number of columns and rows and the cell size to define the location of each individual cell

Question 34

Raster Data Structure

Answer 34

different spatial features must be stored as separate data layer – if you wanted to display water features, land cover, land ownership, and elevation data, you would need 4 separate rasters (remember, rasters can only store 1 value in each cell)
▪ the file size of the raster is defined by the size of the raster, not by the number of features it contains, so a simple raster of 0’s and 1’s will occupy the same file space as a raster with values ranging from 0 to 20

Question 35

Techniques for compacting raster data files: 3

Answer 35

1. Run Length ecoding
2. Block Encoding
3. Chain Encoding
4. Quadtrees

Question 36

Vector Data Structures: 2

Answer 36

▪ vector data structures
▪ how vector data are stored is an important component of understanding the link between digital data and the display
1.consider this simple polygon: spaghetti file
2. Point Dictionary

a problem with the spaghetti and point dictionary structures is that each defined polygon exists in an isolated space – there is no topological context to say that polygon A shares a boundary with polygon B – therefore the GIS cannot identify these polygons as neighbours
▪ topological data structures provide further information regarding the relationship between different polygons

Question 37

_______ model good for simple vector data structure but for more complex ________ model

Answer 37

spaghetti

Point dictionary
-eliminates redundancies that would exist in spaghetti model

Question 38

there are many different topological data structure used in GIS, but they all try to make sure that: 4

Answer 38

▪ no node or line segment is duplicated
▪ line segments and nodes can be referenced to more than one polygon
▪ all polygons have unique identifiers
▪ island and hole polygons can be adequately represented

Question 39

Issue with converting data:
Vector–> Raster
Raster–>Vector

Answer 39

▪ data can be changed between the raster and vector models
▪ however, this often comes with a loss of information and/or data (raster data only contain 1 attribute, whereas vector data can contain many)
-losing accuracy

Question 40

Surface Models

Answer 40

the modelling of surface entities such as height, pollution, or rainfall poses interesting problems in GIS

to model a surface perfectly, you would need an infinite number of data points; a surface model approximates a continuous surface using a finite number of observations, each with a known location
▪ but how many points are needed? The resolution of a digital terrain model (DTM) is determined by the frequency of observations used
▪ DTMs are created from a series of either regularly or irregularly spaced (x, y, z) data points, where x and y are horizontal coordinates and z is the vertical or height coordinate

-You can model a surface in raster or vector(isolines) form(most often raster)

Question 41

Main Raster Surface Model

Answer 41

raster surfaces are commonly called digital elevation matrices (DEM), and is just a grid of height values in which each cell contains a single value representative of height of the terrain that is covered by that cell
▪ the accuracy of the surface depends on the complexity of the terrain surface and the spacing – resolution – of the grid
▪ fine grids are needed in complex terrain, while coarse grids can be used in relatively flat terrain

Question 42

selecting an appropriate DEM resolution is important because

Answer 42

as the grid gets finer the file size and computing time increase

Question 43

Main Vector Raster Model

Answer 43

in its most simple form, a vector DTM mimics a raster DEM by producing a regularly spaced grid of points, each with a known height
▪ a more advanced and complex but more common form is the triangulated irregular network (TIN)
▪ the TIN method joins the height observations together with straight lines to create a mosaic of irregular triangles
▪ the vertices of the triangles represent peaks and depressions while the edges represent valleys and ridges

-the surface of each triangle provide area, gradient, and aspect attributes

Question 44

Main Advantage of TIN as a surface model

Answer 44

▪ the main advantage of TINs is efficiency of storage, since only a minimum number of significant points need to be stored to reproduce a surface
▪ a TIN does not use all of the data points available to it; instead the model process identifies “surface significant” points based on their spatial relationship with their neighbours – points with high spatial autocorrelation are omitted and thus the file size can be minimized