Buffering
| Buffering must cater for:
|
|
|
Describing a location on the Earth's surface
- Projection (assumes a spherical earth). ONLY a mathematical method for drawing a 3D object (the earth) on a 2D surface (map, computer screen).
- Datum - a modification on a sphere - a spheroid. In other words, a mathematical representation of the earth's shape
- Geoid - even more detail than a spheroid. It's an irregular surface.
- Coordinates - locations. Most common include lat/long (geographic), UTM, and State Plane.
More on Datums...... First in common use in the US was Clarke1866. Following in chronological order are NAD27, GRS80, NAD83, and WGS84. Everything from 1980 on are based upon the center of the earth. Those earlier are based on a point on the surface of the earth. To give a little info on the significance of a datum, there is up to a 300 meter difference in x,y coordinates between the NAD27 and NAD83 datums.
More on the Geoid - the definition of the shape of the earth is the field of Geodesy. In theory, the geoid runs through sea level and is a representation of the gravity field of the earth. Far more accurate than mere datum measurements.
Displaying Map Projections - A map projection is a mathamatical method for projection the surface of a globe onto a sheet of paper (3d - 2d)
Only a few (maybe about 20) are in actual use, although there are hundreds out there.
distortion:
- there is always distortion (period)
- there are different types of distortion
- there are different degrees of distortion dependent on where you are
on the map or which type of projection that you select.
Map Classification
Describing the different projections (2 methods)
1) by geometric construction
- conic: projected onto a cone
- cylindrical -- projected onto a cylinder
- azimuthal - projected onto a single surface
- modified by aspect
- equatorial
- polar
- oblique
- modified by case
- tangent cone touches earth along one line -- no distortion along this line
- secant two lines -- have two lines along which there is no distortion
- Polyconic Do a number of cones and sort of stack them up
2) by Preserved properties:
- Area: correct relative size (equal area or equivalent projections) cyl. equal area, sinusoidal, mollweide, eckert IV
- angle: correct shapes. Note that area and angle are mutually exclusive. (conformal Projections). Mercator, lambert conformal conic
- Distance: distances between points are correct (equidistant). Stereographic, gnomonic
- Azimuth: Great circles are straight lines. Often viewed with the pole at the center.
- Compromise Nothing is preserved, but nothing is super distorted. Robinson
Note that both distance and azimuth preserved projections only
apply to or from the center of the map projection. Also, they usually only
show 1 hemisphere or half the earth. gnomonic
These can be modified by interruptions
Data Types and the NSDI
Data Input
- the number 1 bottleneck in GIS applications. Often 80%+ of the project cost
- Need to automate data input process, but that causes problems.
Common problems/considerations include:
- map projections, scale, coordinate system
- raster/vector conversions
- paper distortions
- error corrections
- control points
- discrepancies across map sheets
- user fatigue and boredom
- Modes of data input include the keyboard, scanners, direct conversion from other digital data, and voice input
Socio-Economic data
- Generally include: (may be aggregate or disaggregate)
- demographics
- housing
- migration
- transportation
- economics
- retail
- Sources of Socio-economic data
- field surveys
- government statistics
- government administrative records
- other stuff, including marketing info, mailing lists, etc.
- Issues in using socio-economic data
- cost
- documentation
- data quality
- data conversion
- aggregation
- accuracy of location