Lecture 3 - Georeferencing I Flashcards
What is a map?
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
What is the process of map making?
- Selection of few features that need to be included
- Simplification: involving elimination, aggregation, and smoothing (ex. of a jagged coastline)
- Exaggeration/displacement of included features that are too small to show at scale (ex. highways)
- Classification/attribute data manipulation to reduce complexity
- Symbolization: to represent different classes of features chosen
How are maps and GIS related?
- 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)
What are the 2 broad classes of maps?
- Topographic map
2. Thematic map
What is a topographic map?
- a reference map tool showing many natural/cultural geo features (ex. roads, water, elevation, vegetation, toponomy (labels))
- ex. contour lines
What is a thematic map?
- displays spatial patterns of one theme or set of attributes (communicates geographical concepts)
What are the 3 types of thematic maps?
- Choropleth map: uses zone to show data, like income, education, etc. (boundaries usually human imposed)
- Area class map: shows zones of constant attributes, like vegetation and soil types (boundaries are mixed and messy because they’re natural)
- 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)
What is map scale?
Scale: ratio b/w distances on map and corresponding distances in the real world (ground distance)
How can scale be represented?
- verbally: 1 cm = 1 km
- fraction: 1:100,000 (proportion)
- graphic scale: line bar scale
Describe the difference between large-scale and small-scale maps
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
What is map generalization?
- uses judgement in the elimination of detail/reduction of features
- scale controls the degree of generalization (influences features shown and simplification)
What is the geometric process of map making?
- methods of mapping ellipsoidal or spherical surfaces that are used to represent the earth
What is required to locate phenomena on earth?
- 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
Explain latitude
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
Explain longitude
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
Explain the prime meridian
- arbitrarily assigned by humans
- division b/w E and W hemispheres
- passes through Greenwich
What is a graticule?
- network of lines on a globe or map that represents lat and long
- origin is at equator and prime meridian (0,0)
Explain how lat and long are measured.
- in: degrees° minutes’ seconds”
- 60”=1’ and 60’=1°
- 1 second is approx 100 ft
- decimal degrees is best for GIS
How do you convert from degrees, minutes, and seconds to decimal degrees?
dd = d° + m’/60 + s”/3600
- make sure to use appropriate sign!
- NE: ++ SE: -+ SW: – NW: +-
- always carry at least 6 decimal places
What is a grid?
a set of uniformly spaced straight lines intersecting at 90° angles
- use a grid where graticule is not high enough resolution (ex. small countries)
What is the shape of the earth?
- 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)
What is the geoid?
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
What is the surface composition of the earth?
- 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
Which spheroid do we use?
- wide variety depending on purpose
- ex. WGS84, Everest, etc.
What is geodesy?
Process of measuring position (horizontal control) and elevation (vertical control) of points on earth’s surface (spheroid)
What is geodetic surveying?
- used to establish location of survey monuments
- incorporates earth’s shape/curvature
- good for large areas (ex. world)
What is plane surveying?
- assumes earth is flat
- good for small geographic locations
- uses survey monuments as starting points
What is a datum?
- 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
What is the difference between a horizontal and vertical datum?
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
What is the difference b/w local and global datums?
Local: designed to fit specific area
Global: origin at centre of mass of earth; shape and size best possible approximation for geoid
Briefly describe the history of North American Datums
- 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
Briefly describe the history of global datums
- demand for global reference ellipsoids
- World Geodetic System 1984 (WGS84) most recent
What equation is used to calculate orthometric heights?
Orthometric Heights = Ellipsoid Height +/- Geoid Undulation
H = h +/- N
