13: Field Data Collection Flashcards
Other types of field data
borehold logs
door to door surveys
water quality
climate data
What is surveying?
Process of taking measurements and performing calculations to quantitatively define the spatial location of a series of points and therefore define the shape of a given landscape, feature or structure
alternative: taking field measurements to digitise the environment
Field survey methods
Tape & graduated staff Abney level Quick-set level GPS Total station LiDAR SfM
Typical surveying applications in geography
Coastal geography - how do coasts adapt to weather and climate events, how does sea level change?
Straight line transect (beach profile) compared year to year
Topographic survey - digital elevation model
Kinematic study - glacier or landslide movement
Precision vs accuracy
Precision - more repeatable
Accuracy - closer to the true value
Often in surveying higher precision is better than higher accuracy as all points can be adjusted for the same offset
Absolute vs relative accuracy
Absolute/real world accuracy - accuracy in relationship to an established coordinate system using LINZ benchmark network
Relative
-accuracy in relationship to an arbitrary local reference
Absolute vs relative accuracy
Absolute/real world accuracy - accuracy in relationship to an established coordinate system using LINZ benchmark network
pro: easier for someone else to take their own measurements from the same benchmark to compare
con: can be harder to have access to one if working in remote areas
Relative
- accuracy in relationship to an arbitrary local reference
pro: useful if you want to get detailed in a small area that might be dynamic or far away from a benchmark, you might setup your own benchmark
con: not easy to compare with others
Benchmarks
Reference points
- should be stable and durable
- easily found
- position in good view (area, sky, high point)
Vital to monitor change or establish absolute position
Ideal surveyed (NZTM) and publicly listed (LINZ)
- several levels of accuracy
- some instances ok if not surveyed (relative change)
Two BMs close by can help accuracy of transects
Surveying constraints
- time vs spatial resolution / extent
- safety
- productivity and accuracy
- physical conditions and methods used
Maximising data quality
-reduce sampling where slope is uniform
Maximising data quality
- reduce sampling where slope is uniform
- increase sampling where slope is changing
- always think: how will this look if plotted on a computer
- goals should always inform sampling decisions
- use site diagrams/photos to check your completed results
- safety should never be compromised
- use of non-contact methods for unsafe spots
Sources of GPS error
Obstruction - trees, buildings, mountains
Multipath - could get a signal that has bounced off something else
Satellite geometry - PDOP (position dilution of precision) poor satellite positioning/angular resolution when satellites are too close together
Atmospheric - delays the signal
What is in a GPS signal?
L1 carrier
Code
Navigation message
-Almanac: schedule for satellites so the GPS unit calculates position faster
-Ephemeris: control station positioning of satellites
What is in a GPS signal?
L1 carrier Code Navigation message -Almanac: schedule for satellites so the GPS unit calculates position faster -Ephemeris: control station positioning
Defining GPS accuracy
autonomous <15m
67% confidence interval
value stated for horizontal
vertical accuracy is 2-5x worse
