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
Post-processed differential correction
- setup a base station with survey total station over a known benchmark
- can check error by looking at how far off our measurement is from the known location (offset)
- apply the same offset to measurements taken with the rover at the same time
can apply in real time or post-processing
Types of GPS
Autonomous (15-5m)
D-GPS code correction (2m)
D-GPS carrier correction (0.2m)
Survey grade (>3mm)
Feature logging
Can record point, line and area features in real time. You can record attributes about them at the same time as well.
What is LiDAR?
Laser Distance and Ranging
Airborne and terrestrial
A laser sends out a beam and is connected to other sensors: pitch, heading, inclination + survey grade GPS
Pros: possibility of multiple returns (can filter out vegetation)
Fast with millions of points and cm/mm accuracy
Cons: very expensive!!
What is a total station?
Combined Theodolite & EDM (laser)
Theodolite measures angles, laser measures distance (Slope distance), from these can use algebra to work out vertical and horizontal distance
Excellent accuracy (cm/mm) and precisions but can be slow
Typical works with a mirror or prism
What is SfM?
Structure from motion photogrammetry
computer modelling technique that reconstructs 3D geometry of feature or landscape from a series of 2D images taken while moving through/around a feature
- feature recognition
- aligns like points between images
- uses math algorithms to determine location of camera and relative positions of identified points
- with basic geometry established, many more feature points are identified
- builds a 3D surface using a TIN & overlays imagery
SfM model accuracy
- accuracy before geo-rectification has no scale
- blurry/smeared things
- can’t interpret tops of trees
- wobble from shadows
- spike points
Key points for GIS
when collecting data:
- use a method fit for purpose
- consider goals carefully before
- follow methods closely to avoid extra error
when using existing data:
- be aware of how it was collected - is it fit for purpose?
- ensure presentation doesn’t overstate accuracy
