Lecture 3 - Georeferencing I

Question 1

What is a map?

Answer 1

A to-scale representation (on a flat medium) of a selection of materials or abstract features on the earth’s surface.

• shows more than just earth’s surface b/c it is loosely used to refer to the visual display of info
• don’t always have to be fully accurate
• geometric distortion is unavoidable in representation

Question 2

What is the process of map making?

Answer 2

1. Selection of few features that need to be included
2. Simplification: involving elimination, aggregation, and smoothing (ex. of a jagged coastline)
3. Exaggeration/displacement of included features that are too small to show at scale (ex. highways)
4. Classification/attribute data manipulation to reduce complexity
5. Symbolization: to represent different classes of features chosen

Question 3

How are maps and GIS related?

Answer 3

• maps are main data source for GIS
• maps are analogue databases
• GIS’ roots are in map analysis
• GIS overcomes many limitations of paper maps
• ex. London Underground map is distorted because we don’t care about actual distances, but the stop orders and interchanges (good use here)

Question 4

What are the 2 broad classes of maps?

Answer 4

1. Topographic map

2. Thematic map

Question 5

What is a topographic map?

Answer 5

• a reference map tool showing many natural/cultural geo features (ex. roads, water, elevation, vegetation, toponomy (labels))
• ex. contour lines

Question 6

What is a thematic map?

Answer 6

• displays spatial patterns of one theme or set of attributes (communicates geographical concepts)

Question 7

What are the 3 types of thematic maps?

Answer 7

1. Choropleth map: uses zone to show data, like income, education, etc. (boundaries usually human imposed)
2. Area class map: shows zones of constant attributes, like vegetation and soil types (boundaries are mixed and messy because they’re natural)
3. Isoline map: shows imaginary surface by means of lines joining points of equal value (contours) - used for phenomena smoothly varying across a map (ex. air pressure, temperature)

Question 8

What is map scale?

Answer 8

Scale: ratio b/w distances on map and corresponding distances in the real world (ground distance)

Question 9

How can scale be represented?

Answer 9

• verbally: 1 cm = 1 km
• fraction: 1:100,000 (proportion)
• graphic scale: line bar scale

Question 10

Describe the difference between large-scale and small-scale maps

Answer 10

Large-scale: shows a small area in great detail (ex. 1:1,000)
Small-scale: shows large area in little detail (ex. 1:250,000)
To tell difference, 1/1,000 is bigger than 1/250,000

Deciding what is large or small is subjective and based on comparisons b/w maps

Question 11

What is map generalization?

Answer 11

• uses judgement in the elimination of detail/reduction of features
• scale controls the degree of generalization (influences features shown and simplification)

Question 12

What is the geometric process of map making?

Answer 12

• methods of mapping ellipsoidal or spherical surfaces that are used to represent the earth

Question 13

What is required to locate phenomena on earth?

Answer 13

• a location system through a grid of lines
• geographic coordinate system: specifies positions on the globe through latitude and longitude (E-W)
• expressed in degrees, minutes, and seconds, or in decimal degrees

Question 14

Explain latitude

Answer 14

It measures angular deviation north or south from the equator along a meridian.

• equator is a reference plane used to define longitude (0 degrees latitude)
• parallels are lines of equal latitude

Question 15

Explain longitude

Answer 15

It measures the angle on the equatorial plane east to west b/w the meridian of the point and the central meridian through Greenwich, England

• prime meridian is reference plane used to define latitude (0 degrees longitude)
• meridians are lines of equal longitude

Question 16

Explain the prime meridian

Answer 16

• arbitrarily assigned by humans
• division b/w E and W hemispheres
• passes through Greenwich

Question 17

What is a graticule?

Answer 17

• network of lines on a globe or map that represents lat and long
• origin is at equator and prime meridian (0,0)

Question 18

Explain how lat and long are measured.

Answer 18

• in: degrees° minutes’ seconds”
• 60”=1’ and 60’=1°
• 1 second is approx 100 ft
• decimal degrees is best for GIS

Question 19

How do you convert from degrees, minutes, and seconds to decimal degrees?

Answer 19

dd = d° + m’/60 + s”/3600

• make sure to use appropriate sign!
• NE: ++ SE: -+ SW: – NW: +-
• always carry at least 6 decimal places

Question 20

What is a grid?

Answer 20

a set of uniformly spaced straight lines intersecting at 90° angles
- use a grid where graticule is not high enough resolution (ex. small countries)

Question 21

What is the shape of the earth?

Answer 21

• not a sphere, but a biaxial ellipsoid/spheroid (irregular)
• not uniform (ocean is smooth while topography varies
• it is a geoid (physical figure of the earth)

Question 22

What is the geoid?

Answer 22

Mathematical figure of the earth’s surface based on gravitational measurements/calculations

• if the earth stopped spinning and everything settled it would look like this
• can’t see it visually, so it is theoretical and modelled

Question 23

What is the surface composition of the earth?

Answer 23

• topographic surface is the interface b/w the land and air and is complex/difficult to model (sharp and bumpy, like mountains and valleys)
• the 3D best fit smooth surface of the geoid is the ellipsoid
• the geoid goes above and below the idealized ellipsoid in certain places

Question 24

Which spheroid do we use?

Answer 24

• wide variety depending on purpose

- ex. WGS84, Everest, etc.

Question 25

What is geodesy?

Answer 25

Process of measuring position (horizontal control) and elevation (vertical control) of points on earth’s surface (spheroid)

Question 26

What is geodetic surveying?

Answer 26

• used to establish location of survey monuments
• incorporates earth’s shape/curvature
• good for large areas (ex. world)

Question 27

What is plane surveying?

Answer 27

• assumes earth is flat
• good for small geographic locations
• uses survey monuments as starting points

Question 28

What is a datum?

Answer 28

• reference mathematical model used for projection to geoid
• formula that combines ellipsoid and its geoid to surveyed control points
• basically, a reference point from which we measure something

Question 29

What is the difference between a horizontal and vertical datum?

Answer 29

Horizontal: precise location known and used as reference point
- make sure selected ellipsoid fits area of interest
Vertical: surface to which elevations are referenced
- ex. point that is 17m below sea level is reference for other buildings

Question 30

What is the difference b/w local and global datums?

Answer 30

Local: designed to fit specific area
Global: origin at centre of mass of earth; shape and size best possible approximation for geoid

Question 31

Briefly describe the history of North American Datums

Answer 31

• 1900: first nationwide datum in US with 2,500 points (centre in Meades Ranch, Kansas)
• NAD27: 25,000 stations
• NAD83: uses satellites and 275,000 stations withe origin at the centre of the mass of the earth
• CANNOT convert from NAD27 to NAD83

Question 32

Briefly describe the history of global datums

Answer 32

• demand for global reference ellipsoids

- World Geodetic System 1984 (WGS84) most recent

Question 33

What equation is used to calculate orthometric heights?

Answer 33

Orthometric Heights = Ellipsoid Height +/- Geoid Undulation

H = h +/- N