10 - Spatial data input Flashcards
Cheap vector data
- Shapefiles lacking topological relationships
- Comprise digitizing errors & attribute errors too
Cheap raster data
- Coarse scale vs fine scale (i.e. spatial resolution)
- Temporal resolution (landscapes change over time)
NGI custodian
- Aerial photos
- National roads
- Main/ secondary roads
- Place name areas
- Land use
- land cover
- Geodesy
SANSA custodian
- satellite
SANRAL custodian
- National roads
PG custodian
- Main/ secondary roads
- place name areas
- Land use
- Land cover
LA custodian
- Main/secondary roads
- Dwellings
- Land use
- Land cover
- Land parcels
Stats SA custodian
- Place name areas
- Dwellings
ESKOM custodian
- Dwellings
NGI custodians
- Place name areas
- Trig beacons, TrigNet
PG custodian
- Land use
- Land cover
- Cadastre
SG custodian
- Land parcels
Cadastre meaning
an official register of the quantity, value, and ownership of real estate used in apportioning taxes.
Geodesy
the science of accurately measuring and understanding the Earth’s geometric shape, orientation in space, and gravity field
Primary raster data
Digital remotely sensed images (satellite, aerial photos)
Secondary raster data
- Scanned maps
- Scanned aerial photos
Primary vector data
- GPS
- Field survey
Secondary Vector data
- Topographic maps
- Data Digitizing
Data encoding (digitizing/ key-coding)
Converting (attribute) data from a non-digital (analog) to a digital format
Measures for map preparation
- Taped securely in centre
- Flat, no wrinkles
- Table adjusted for each reach
- Planning & organization
3 steps of data encoding process
- Geocoding
- Add attributes
- Quality Assurances
Methods of data encoding
- Digitizing (Manual = Tablet & HUD OR automatic stream
mode) - Keyboard, COGO – typing from keyboard (mainly attributes;
occasionally co-ordinates) - Scanning
- Direct file transfer, file conversion
- Remote sensing images
Manual digitizing
Human guided capture of features from a map image or source.
2 methods:
- on-screen digitizing
- hardcopy digitizing
Stream mode
nodes captured at a set rate per time unit – speed at which the cursor is moved determines the number of points captured. (Faster = less, slower = more)
HUD
Heads up digitizing
Disadvantage of automatic digitizing
Need lots of cleaning after scanning is completed; especially removal
of non-target features.
3 types of scanners
- Large-format feed (most common)
- Flat bed (usually too small)
- Rotating drum (high accuracy, slow)
Advantage of automatic digitizing
cost-effective for capturing
very large volumes of data
Process triangulation
Methodological triangulation is the process of using multiple methods of data collection (usually 3 satellites)
GPS definition
A system of earth-orbiting satellites that provide location (in lat/long
coordinates) and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to
four or more GPS satellites
Space segment
Constellation of 24 -32 satellites ➔ Arranged in six equally-spaced orbital planes @ altitude of 20 183km
Control segment
- Monitor stations
- Tracking, monitoring, updating and synchronizing SV’s (Space vehicles)
User segment
- GPS Receivers
- Applications
- Software
Trilateration
GPS receiver measures the distances to satellites using radio signals. Trilateration = distance
Triangulation = angles (using travel time of radio signals)
GLONASS
Russian
Galileo positioning system
EU & ESA (European space agency)
COMPASS navigation system (Beidou-2)
China
IRNSS (Indian regional navigational satellite system
India
GPS vs GNSS
GPS - single satellite system with 31 satellites
GNSS -89 satellites from all 4 systems
The 4 satellite systems
- GLONASS
- Galileo
- Beidou-2
- IRNSS
Accuracy of consumer grade
- 15-30m
Accuracy of mapping grade
-1-5m
Accuracy of survey grade
- sub-m-cm
Consumer grade example
- Garmin
-Magellan;
no post-processing available
Mapping grade examples
- GeoExplorer III
- CMT Mach III;
post-processing or real-time
Survey grade example
Trimble post-processing or real-time
Orbital error
Caused by the satellite oscillator not being synchronised to true time (GPS time).
Satellite-based uncertainties
- Clock (orbital) error = 1 m
- Positional error (ephemeris)
Ephemeris def
Tabulation of computed positions and velocities of an orbiting body at specific times.
Errors due to signal propagation (scrambled signal)
- Atmospheric delay error = up to 11m
- Multipath error (buildings) = 0.5 m
Multipath error
Caused when direct signals from satellites are mixed with those reflected from objects in the vicinity of the antenna
Received-based uncertainties
- Clock bias
- antenna centre
- noise
Clock bias def
The weakness of gravity makes the clocks in the satellites appear to run faster than the clocks on earth by about 45 microseconds a day.
Antenna (phase) centre error
- APC does not coincide with the antenna’s physical center
- Varies with elevation, azimuth, intensity of the satellite, and frequency of the incoming signal.
What causes noise errors?
Malfunctioning or incorrectly configured transmitters,
Error measurements
- PDOP: Position Dilution of Precision (3D)
- TDOP: Time Dilution of Precision
- HDOP: Horizontal Dilution of Precision (Latitude and Longitude)
- VDOP: Vertical Dilution of Precision (Height)
GPS for SA
- Datum = WGS84
- Reading (position) format = DMS
- Projection = Geographic
- Country = South Africa
- Time Zone = Other
- UTC offset = +02:00
- Time format = 24 hour
- Units = metric
- North ref = true
- Direction display = Cardinal Letters
- 12 channels, has a reading-averaging function, and
- can export data through a download cable
Temporal resolution
The amount of time
needed to revisit and acquire data for the exact same location.
- high TR = revisiting delay is low
- low TR = revisiting delay is high
Data validation
- Attribute accuracy
- Logical consistency
- Completeness
Attribute accuracy
▪ Wrong value in an attribute field
▪ Missing attribute
▪ Misspelled work or other typographical error
Fix: Sort attributes (~ Excel and check visually) & Run queries
Logical Consistency
▪ Does the data make sense ?
▪ Do water pipelines connect at nodes?
(logical interpretation and validation of spatial data)
Completeness
▪ Are all land-use parcels represented in the parcel layer?
▪ Is there any portion of the data missing ?
