GIS Section 2 Flashcards
Studying for Intro to GIS midterm!
What is georeferencing?
process of converting positional data of objects and their elements to reference systems relative to a real position on earth
Why is positioning important?
accurate referencing of a location is the basis for analysis, management, and reporting of geographic data
What happens if there is more than one referencing system in a GIS project?
- positions must be converted/transformed/projected from one system to another
Why might GPS positioning data be inaccurate?
could be obtained by someone with little knowledge of underlying GPS, or from inaccurate/cheap equipment
Why is high-precision GPS data hard to to obtain?
requires specialized equipment and technicians with specialized knowledge
What types of positioning systems are there, broadly?
- global systems
- local systems
- discrete systems (ex. postal codes)
What are the components of a georeferencing system?
- coordinate system
- ellipsoid of reference (geodetic datum)
- projection type
- transformation equations
What are the two general types of coordinate systems?
- polar coordinate systems - centered on a point of interest
- 3D Cartesian coordinates - centered on center of the Earth
What is an ellipsoid of reference/geodetic datum?
ellipsoid models (approximations) of the Earth’s surface
- account for flattening at poles (around 20km difference between sea level and level of a perfect sphere)
- best ellipsoid models represent the Earth’s shape within ~100m
- changing between coordinate systems also requires shifting the datum
What are the components of global coordinate systems?
- latitude (phi) (-90 to +90)
- longitude (lambda) (-180 to +180)
Accuracy vs. Precision?
Accuracy: reflects how close a measurement is to a known/accepted value
Precision: reflects how reproducible measurements are, regardless of how close they are to an accepted value
What is geographic projection?
formulations for projecting positions of objects onto a flat plane
need to maintain some properties:
- shape
- curve
- distance
- area
- direction
What properties might map projections aim to maintain?
- shape (conformal/orthomorphic)
- curve
- distance (true distance= not possible)
- area (equal area/equivalent)
- direction (compass-bearing mercator gnomic projection)
What are the main projection families?
- cylindrical
- conical
- azimuthal (planar)
What kind of projection is UTM?
cylindrical
How wide is each UTM zone?
6 degrees
How are positions measured in UTM
meters eastward from the central meridian of the zone, and northward from the equator
What is discrete georeferencing?
positioning by recorded street address/postal code - convenient because existing addresses can be automatically converted to GID database
the digital record for the feature must have a field linking it to a geographic base file/position
What are the two kinds of coordinate transformations?
Affine - linear equations (6 coeffs) - keep parallel lines parallel
Curvilinear -
What can you do with Affine transformations?
- translation
- scaling
- usually a combination is needed (with coefficients calculated through multiple linear regression)
What are curvilinear transformations, and why use them?
complex transformations (ie. at higher powers)
- create curved surfaces (rubber sheeting)
- give GREATER ACCURACY!
What transformations are often used to georeference?
first-order polynomial equations (affine) - needs at least three control points
What is the drawback of higher-order transformation equations?
creates more complex distortion
What is a GCP in georeferencing?
Ground Control Point
What is resampling?
methods for determining digital values of pixels placed in new locations
What are the three resampling methods?
- nearest neighbour (uses value of pixel of original image that is nearest to the corrected location) (blocky appearance!)
- bilinear interpolation (weighted average of 4 pixels)
- cubic convolution (weighted average of 16 pixels)
What is the GPS?
earth-orbiting satellite system (24 GPS satellites), provides signals available anywhere on/above earth
- funded/controlled by USA DoD
- increasingly-used inpput for GIS
- effective/accurate use requires some training
How does GPS work (satellites)
- 24 satellites send radio signals from space
- 6 orbital planes
- between 5-8 SV (space vehicles) visible from any point on Earth
What is the control segment of GPS?
a system of tracking stations around the world (measure SV signals, compute orbital/location data, upload data to SVs which the SVs can send to GPS receivers)
How does the user segment of GPS work?
- GPS users and receivers - convert SV signals into position/velocity/time estimates
- 4 satellites required over the horizon to compute the 4 dimensions (x,y,z, time)
- each SV (space vehicle) has its own repeating C/A code - modulating pseudo-random noise (PRN) that can be used to distinguish individual SVs
What are the 2 levels of positioning service by GPS?
- 2 types of signals
- Standard Positioning Service (SPS) - general civilian use, a few m of accuracy
- encoded Precise Positioning Service (PPS) - intended for military use by DoD, scrambled to prevent enemies from targeting it, pinpoints location within 3ft
