GPS, Remote Sensing and Digitizing

Question 1

Spatial collection methods

Answer 1

Vector:
SHP from internet
Coordinates from database file
GPS
Digitizing - various sources
Address geocoding

Raster:
Free downloads
Purchase

Question 2

Global Positioning Systems

Answer 2

A system that allows for calculation of position with high degree of accuracy
Based on a ground receiver and a system of 24 satellites orbiting earth and using a process of triangulation - need at least 3 satellites
The arrangement of GPS orbiting satellites is called constellation - minimum constellation: 24 (plus 7 more for back up)
4 unknowns: latitude, longitude, elevation (Z) and time (T)

Question 3

Satellite Geometry

Answer 3

A good satellite geometry is KEY to achieve precise position readings
Geometric dilution of precision (GDOP): A measurement of the effect of satellite geometry on GPS precision
1-3 excellent, 4-6 good, 7-8: moderate → want a lower number - a high score the less accurate
Want nice angles - diverse not clustered

Question 4

GPS Sources of Error

Answer 4

Atmospheric effects in the troposphere and lonosphere
Error in the synchronisation of satellites
Reflection of satellite signal before it reaches the receptor
Uncertainty on the real position of satellites (ephemeris error)
Satellite geometry

Question 5

May 1, 2000

Answer 5

As of May 1, 2000: government stopped adding additional random error components to GPS signals. This makes inexpensive units more accurate
95% errors will be <20m v <100m previously
*Do point averaging if your receiver allows - 60-120 counts is good and will help minimise error

Question 6

Condo Canyon

Answer 6

buildings and trees can block receiver - might have to take a pseudo-point if can plot the distance and direction of the correct post

Question 7

Common GPS Mistakes

Answer 7

Remember when writing down coordinates that you record ALL the digits - at least 4 digits
Don’t forget when using degrees, minutes and seconds to record NS, EW or +/- units

Question 8

Converting latitude and longitude coordinates

Answer 8

= degrees + (minutes/60) + (seconds/3600)

If south or west add a - sign

Question 9

Digitizing

Answer 9

creation of a spatial dataset from a hardcopy (paper map) or digital (on-screen) source
To generate base maps when online information not available
Can save lots of data collection time
Google Earth allows to digitise and export points as kml

Question 10

Digitising Caution

Answer 10

Be careful to make sure you zoom in enough on your imagery
Otherwise you might not be placing your points lines or polygons where you think you are placing them
This introduces error and is especially an issue if you need to conduct spatial analysis