NGEA31
From where do we obtain geographical data?
Measurements in the field
Digitized maps or images
Interpolations from point sources
Estimates from out of the blue
What does geographical contain?
Geometric data (x and y coordinates) and attribute data
What is a vector data structure?
point, lines and polygons
How is vector data strucutred?
They are stored in a file of database
What are the characterstics of points?
Dimensionless
A point has one pair of X Y coordinates
Each point have a unique ID-value
What are the characteristics of line features?
Have one dimension
Consist of ordered coordinate pairs
Each line segment has a start node and end node
Inbetween one or more vertices (breakpoints)
Attributes valid for the segment is linked through the ID value
What are the characteristics of polygon features?
Created by a line that enclose the polygon area
Have two dimensions, X and Y
Expect homogenity inside
What is a shape file?
Has a simple structure
Non topological
Stores geometrical location and attribute information in seperate files
Stores the feature geometry
What does a .dbf store?
stores the attribute info
What is .prj?
reference system
What is .shx?
link between geometry and the attributes
Name the issues with the simple polygons model
Storage problems (boundaries are stored twice)
Neighbor searches complicated (entire list of coordinates are searched until an identical pair is found)
Island polygons are hard to handle
What happens if you add a coordinate list to a simple model?
The shared border coordinates are stored once, but still problems with island polygons
What is topology?
knowledge of the geometrical relationships and connectivity between objects
Explain topological polygon structure
Shared lines stored once
Island polygon: easier to find
Different tables for describing different aspects of the topolgy
Which polygon structure should you use?
Simple polygon if only drawing maps
Topological polygon structure for effective spatial search and spatial analysis
What does SQL mean?
It is used to analyse numerical data in databases
The purpose is to perform queries in a well defined structure
Give the syntax of SQL
Select (columns)
From (which table)
Where (criteria)
Explain the OR criteria
Either alternative works
Explain AND criteria
both criteria needs to be fulfilled
What is a GIS?
Geometrical objects (vector or raster)
With attribute objects (more information about the object)
Name the different systems
Flat file system
Hierarchial file system
Network file system
What are Relational Databases
Data organized in tables
Uses 'keys'
Name the 4 types of relationships
one to one (one file)
one to many (hierarchial = one parent many children)
one to many, many to one
many to many (many tables can be related in many different ways)
What are the rules for Relational databases?
Avoid redundancy
For each table we must define name and column
How do you solve redundancy?
Make many small tables
Relations are made when needed
What are the two data models?
Hybrid storage (stores spatial data and attribute data seperately, the format is shape format)
Object based data model (combines geometries and attributes in one system, format is geodatabase)
Nominal is....
e.g. name, gender, specie
Ordinal is....
Rank ordered, totally agree/mostly agree
Interval is....
Temperature, aspect
Ratio is...
Distance, area, income
Which symbols are there?
Point, line, area
What do you do for choropleth maps?
Normalize data and use single color grading from light to dark
Name the rules for map design
selection of mapping method
classification of data into groups
selection of colors and symbols
labelling attributes
Name the classification methods
Natural breaks
Equal intervals
Quantiles
Standard deviation
Population density
What is hue?
basic colour we perceive, eg 12 steo wheel
What is value?
lightness or darkness - Can be hard to perceive variations in value
What is saturation?
intensity or purity compared to a neutral grey
Name the common generalization techniques
simplification
smoothing
aggregation
merging
collapsing
displacement
What is the first step generalization?
Base layers
Second step of generalization
Thematic map
Name the 4 basic geometric vector operations
Computation of distance b/w 2 features
Calculating length of line objects
Calculating polygon areas
Calculating line intersections
What is the Euclidian Distance
as-the-crow flies = Pythagorean Theorem
What is distance in a network?
sum of segments b/w start and end node
What is topographical Distance?
DEM analysis
What is spherical distance?
airline routes
Which 3 vector overlays are there?
Points in polygons
Lines in Polygons
Polygons in polygons
Explain how points in polygons work
Decide which points lie in which polygon
Transfer the attributes from the polygon to the points that fall within them (one-to-many)
Result: point layer with their original + polygon attributes
First step line in polygon
Find intersection between lines and polygon borders
Second step lines in polygon
Lines divided into new objects
Third step lines in polygon
Decide which new line fall into which polygon
Fourth step lines in polygon
Transfer attributes from polygon to corresponding new line objects
What is not automatically updated in shape file format?
Length and area
What is a common problem with polygons on polygons?
Slivers
What are the post overlay clean-up operations?
Clip
Dissolve
Merge
Eliminate
Explain clip
Same principle as polygon on polygon overlay but no attribute from clip layer included in the table
Explain dissolve
Uses attribute values to aggregate unit polygons into new, larger polygons which contain at least one common attribute from the smaller polygon
Explain eliminate
To remove slivers. Used to eliminate polygons with an area less than a certain threshold value.
Explain merge
Bringin together two adjacent data layers in order to create a larger database = map matching
What influences vector analysis?
Map projection
Generalisation
Precision (data type)
Which data format goes with the hybrid data model?
Shape file
Which data format goes with the extended data model?
Geodatabase
What is the definition of the sphere?
A three dimensional surface
All points on the surface have an equal distance to the center
Definition poles
end of the rotational axis
Definition Equator
An imaginary line on the surface halfway
Definition meridians
Halves of great circles that all come together at the poles
Definition longitudes
the angle between the plane of the prime meridian and that of the meridian through a point for east/west direction
Definition paralles
Imaginary lines parallel to the equator
How do we define the shape of the earth?
We use the rotational ellispoid model
What is datum?
ellipsoid + where to place the ellipsoid in relation to earth
Datum can be ... or ... centered
earth or local
Describe earth centered
Centre of ellipsoid = centre of earth's mass
and suits fairly everywhere
Describe local centered
Surface of ellipsoid
very suitable in some location, in others not
What is geoid
equipotential surface in the Earth gravity field.
Geoid height =
geoid - ellipsoid
Orthometric height =
height above sea level/geoid
Ellipsoidal height =
height above the ellipsoid
What is a geodetic reference system?
system that helps us to define a location/spot on earth
What defines a geodetic reference system?
The ellipsoid model
A location of ellipsoid
A coordinate system
Desribe Azimuthal projection
The plane touches earth at one point. Good for small areas and for visualisation
Describe the three cylindric projections
Normal: cylindric touches the equator
Transverse: cylindric touches the prime meridian
Oblique: cylindric touches a great circle somewhere else
Conic projections
Normal: the cone touches the globe along a parallel
Oblique: cone touches the globe anywhere
What distortion is not possible?
zero
Where is the most distortion?
The furhter from the prime meridian
Scale factor = 1
local scale equals principal scale
Scale factor < 1
Local scale is reduced
Scale factor > 1
Local scale is increased
What is a equal area projection
Preserves the relative sizes of geographic features. Distort the shape of features
Conformal projection
Preserves the local shapes. The relative size of geographic features changes.
Where is the projection correct?
Standard meridian
What do we need to go from 3D to 2D?
A suitable projection model
How/where to locate the model
Apply a plane coordinate system
What does the standard meridian define?
It defines where the cylinder touches the earth model. Also defines the origin of the plain coordinate system.
Measurement method: Transverse
to estimate the coordinate pair of a point when you know the angle and distance to a point with known coordinates.
What are the main segments of GPS?
Space segment
Control segment
User segment
How many satellites needed to estimate the exact position of location?
4
Possible to obtain coordinates with 3 satellites?
Yes, GPS uses its memory and knowledge about the earth as a sphere to cancel one of the points. But accuracy is better when using 4 satellites.
Name the errors conncected to GPS measurements
Ephemeris errors (difference between expected and actual location of satellite)
Troposphere/ionosphere/atmosphere
Clock errors
Environmental disturbance
Multiple path reflection
What to consider in field regarding measurements?
At least 4 satellites
Satellite configuration (DOP)
Reduce risk for multi path reflection
Time for GPS to stabilize
What is a raster model
The represented surface, e.g. the elevation in an area, is divided into a number of raster cells
Cell =
Pixel, the building stones of a raster
How do you calculate the number of cells?
n (total of rows) * m (total of columns)
What is spatial resolution?
Smallest distance where a change can be observed.
What does a high spatial resolution mean?
raster with cells equal to a small area on the ground.
smaller details can be detected
How do you calculate spatial resolution?
cell width x cell height
What happens if you increase the resolution?
storage space needs to be increased
What problem occurs with representing vector data as raster data?
Precision problem. Impossible to represent the placement within a raster. Solution would be to increase the resolution at the cost of memory space.
What information is necessary to store with a raster?
X and Y coordiantes
Number of columns and rows
Attribute table
Resolution
Raster analysis methods are:
Local
Focal
Zonal
Global
What does AND operator mean?
Both criterias must be fulfilled
What does OR operator mean?
Max value
When is substracting used?
Substracting one raster from another is used to detect change
What is a focal operator?
it involves a focal cell and set of surrounding cells chosen for their distance and/or directional relationship to the focal cell.
Name the 4 common neighbourhood types
Rectangle
Circle
Annulus
Wedge
What is focal majority
The output focal cell is the most frequent value occuring
Focal mean
The output is the mean of the focal cell and the neighbour cells
Why change individual cell values on the basis of neighbouring cell values?
To smooth continuous surfaces
Remove speckle
Estimate form parameters
Enhance changes
What is Aspect?
it is the direction of slope in degrees
What does a zonal operation do?
it measures the geometry of each zone in the raster (given a single input raster)
Topography can be presented both in
Vector and Raster data
Topographical data in raster form ....
Are often created by interpolation
Give, depending on cell size, a good representation of the surface
Are memory demanding
What are the difficulties with DEM?
Sinks and flat areas
Why are those problems occuring with DEM's
Sinks and flat areas are a problem in hydrological modelling
When and how do you use estimation of volumes?
When: if we want to build a road for example and estimate cutting and filling volumes
How: Check the differences in elevation between the cells and the planned road multiply by cell size
How is a slope estimated?
By the use of a filter operation, normally a 3x3 cell filter is applied
What is drainage direction
The flow direction of non blocked surface water and equals the aspect value
The results of all DEM/DTM operations are VERY sensitive to...
Cell size
Sampling and interpolation
Estimation of an unknown value by the use of known surrounding cells
What is TIN
Vector representation as a triangular irregular network
What does the cell value represent
It represents the elevation in the centre (point) of the corresponding surface
What types of sampling are there?
Systematic
Random
Stratified random
Purposeful
What is the first local interpolation?
Thiessen polygons: an unknown point gets the value of the closest known point.
What is the second interpolation?
IDWA: the closer to a known data point we are, the more it influences the estimation.
Which question arise with IDWA interpolation
How many known data points to use
Maximum distance to known data point
Distribution of known data points
What is global interpolation?
Method that uses every known point available to estimate an unknown value.
What is local interpolation?
Uses a sample of known points to estimate an unknown value
What is exact interpolation?
It predicts a value at the point location that is the same as its kown value. So, it generates a surface that passes through the control points.
What is inexact interpolation
predicts a value at the point location that differs from its known value
What is deterministic interpolation?
a method that provides no assessment of errors with predicted values
Stochastic interpolation
This method considers the presence of some randomness in its variable and offers assessment of prediction errors with estimated variancs