Principals of GIS Flashcards
What is scale factor? Know some examples.
Scale factor = ratio of local scale/primary scale. So, scale factor is the ratio of local scale and the primary scale. The local scale is the scale of the map and the primary scale is the scale of the globe. When the scale factor = 1, the local scale equals the primary scale and there is no map distortion. The scale factor = 1 at the standard line. The standard line is in essence where the projection surface touches the reference globe.
describe porjection
the process of transforming the spatial relationship of features on the Earth’s surface to a flat map. The three important projections are cylindrical, conic and planer (azimuthal). These projections can touch the reference globe at one point or line which is a simple projection. At two points it is known as secant. Different projections preserve different things: Conformal (Shape), Equivalent (Equal Area), Distance and Direction. None can do both preserve shape and area.
describe coordinates
numeric value that fixes the location of a point in space and links that point to the Earth’s surface.
- -Geographic Coordinates - These are lat and long. Geographic coordinates are the most familiar. The draw back is that the coordinates are based on a globe so values are in degrees. Latitude is in the angle from the center (equator) above and below. Longitude is the angle from the 0 line which goes through London.
- -Plane Rectangular Coordinates - This is a 2 dimensional Cartesian grid. Units are in meters and are specified in x and y coordinates. The units are somewhat easier to understand than for geographic coordinates.
- -Plane polar Coordinates - gives an angle and a distance (don’t think this was really discussed in the class)
Describe Datum
Mathematical model of the Earth which serves as the reference or base for calculating the geographic coordinates of a location. It links the reference ellipsoid to the true shape of the earth.
- -Geodetic datum= horizontal position
- -Vertical datum = altitude or elevation
Describe Reference Ellipsoids
are used to model Geoid (flattening ratio = (a-b)/a, semi-major axis length =length of a)
- Used in map making to represent the surface of the earth
- No reference ellipsoid accurately portrays the entire globe perfectly
- Distortions exists
- Each regions has a different reference ellipsoid
- Distortion is minimized where the ellipsoid mirrors the actual earth surface
- Some commone referrence ellipsoids are the NAD27 and NAD83
List attribute of Vector data.
Vectors are points, line or polygons. They represent discreat features. They are object based with identifiable boundaries. Vectors use topology Vectors can be very precise. Vectors have a well defined attribute table. Visually appealing. Small file size Good spatial accuracy
List attributes of Raster data.
Units are grid cells. Blocky. No clear boundaries No topology no organized attrbute table They represent continuous data does not have discreet objects. Each cell is homogenous. These tend to have large file sizes good for modeling.
What is topology in GIS?
Topology=a subfield of mathematics that, when applied to GIS, ensures that the spatial relationships between features and expressed explicitly. Topology tells the computer what the spatial relationship is between features. It is very useful in modeling and understanding things which require flow - how water would flow through a series of pipes, etc…
List the databases and explain each one.
Simple List - unorganized list
Index List - organized simple list (Phone book)
Flat File - Like in Excel. Good for data analysis but not good for large amounts of data
Relational Database - Line in MS Access. Better for large amounts of data. Can perform complex queries. Consists of a series of smaller tables which are linked by a common value. In terms of relationship cardinality, there can be a one to one, one to many, or many to many relationship. Example, one to many would be: one professor linked to many students.
What is relationship cardinality?
In terms of relationship cardinality, there can be a one to one, one to many, or many to many relationship. Example, one to many would be: one professor linked to many students.
Name the types of Queries?
Attribute and Spatial
What are the boolean operators?
Complex queries asking more than one thing at one time. AND, OR, NOT, XOR
What is required for an overlay?
input feature (Point/Line/Polygon) + Overlay (Polygon) = Output Feature with the same geometry as input.
Name the types of overlays.
Point to Polygon (Output is two points with attributes spliced together with overlay feature), Line to Polygon, Polygon to Polygon.
What does Union do
Same areas of both polygons but divides it up based upon borders.
