GIS Section 2

Question 1

What is georeferencing?

Answer 1

process of converting positional data of objects and their elements to reference systems relative to a real position on earth

Question 2

Why is positioning important?

Answer 2

accurate referencing of a location is the basis for analysis, management, and reporting of geographic data

Question 3

What happens if there is more than one referencing system in a GIS project?

Answer 3

• positions must be converted/transformed/projected from one system to another

Question 4

Why might GPS positioning data be inaccurate?

Answer 4

could be obtained by someone with little knowledge of underlying GPS, or from inaccurate/cheap equipment

Question 5

Why is high-precision GPS data hard to to obtain?

Answer 5

requires specialized equipment and technicians with specialized knowledge

Question 6

What types of positioning systems are there, broadly?

Answer 6

• global systems
• local systems
• discrete systems (ex. postal codes)

Question 7

What are the components of a georeferencing system?

Answer 7

• coordinate system
• ellipsoid of reference (geodetic datum)
• projection type
• transformation equations

Question 8

What are the two general types of coordinate systems?

Answer 8

• polar coordinate systems - centered on a point of interest
• 3D Cartesian coordinates - centered on center of the Earth

Question 9

What is an ellipsoid of reference/geodetic datum?

Answer 9

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

Question 10

What are the components of global coordinate systems?

Answer 10

• latitude (phi) (-90 to +90)
• longitude (lambda) (-180 to +180)

Question 11

Accuracy vs. Precision?

Answer 11

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

Question 12

What is geographic projection?

Answer 12

formulations for projecting positions of objects onto a flat plane

need to maintain some properties:
- shape
- curve
- distance
- area
- direction

Question 13

What properties might map projections aim to maintain?

Answer 13

• shape (conformal/orthomorphic)
• curve
• distance (true distance= not possible)
• area (equal area/equivalent)
• direction (compass-bearing mercator gnomic projection)

Question 14

What are the main projection families?

Answer 14

• cylindrical
• conical
• azimuthal (planar)

Question 15

What kind of projection is UTM?

Answer 15

cylindrical

Question 16

How wide is each UTM zone?

Answer 16

6 degrees

Question 17

How are positions measured in UTM

Answer 17

meters eastward from the central meridian of the zone, and northward from the equator

Question 18

What is discrete georeferencing?

Answer 18

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

Question 19

What are the two kinds of coordinate transformations?

Answer 19

Affine - linear equations (6 coeffs) - keep parallel lines parallel

Curvilinear -

Question 20

What can you do with Affine transformations?

Answer 20

• translation
• scaling
• usually a combination is needed (with coefficients calculated through multiple linear regression)

Question 21

What are curvilinear transformations, and why use them?

Answer 21

complex transformations (ie. at higher powers)

• create curved surfaces (rubber sheeting)
• give GREATER ACCURACY!

Question 22

What transformations are often used to georeference?

Answer 22

first-order polynomial equations (affine) - needs at least three control points

Question 23

What is the drawback of higher-order transformation equations?

Answer 23

creates more complex distortion

Question 24

What is a GCP in georeferencing?

Answer 24

Ground Control Point

Question 25

What is resampling?

Answer 25

methods for determining digital values of pixels placed in new locations

Question 26

What are the three resampling methods?

Answer 26

• 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)

Question 27

What is the GPS?

Answer 27

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

Question 28

How does GPS work (satellites)

Answer 28

• 24 satellites send radio signals from space
• 6 orbital planes
• between 5-8 SV (space vehicles) visible from any point on Earth

Question 29

What is the control segment of GPS?

Answer 29

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)

Question 30

How does the user segment of GPS work?

Answer 30

• 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

Question 31

What are the 2 levels of positioning service by GPS?

Answer 31

• 2 types of signals

1. Standard Positioning Service (SPS) - general civilian use, a few m of accuracy
2. encoded Precise Positioning Service (PPS) - intended for military use by DoD, scrambled to prevent enemies from targeting it, pinpoints location within 3ft