What are spatial data sources?
- Spatial data is one of the most important components of a GIS.
- Unfortunately, spatial data is often very expensive.
- In the case that it is cheap or open source the data are often of lower quality.
Vector data:
Shapefiles might be lacking topological relationships
Comprise digitizing errors + Attribute errors too
Raster data:
Coarse scale vs fine scale (i.e. spatial resolution)
Temporal resolution (landscapes change over time)
what kind of source can the data be?
Primary - not processed = raw data
Secondary - processed data / from another party.
What is data encoding? (digitising)
converting data from no -digital to digial format.
What are the three steps to data encoding process?
- Geocoding - capturing co-ordinates for the observation.
- Add attributes - what do the data points represent?
- Quality assurances - checking that data has been captured accurately and precisely.
What are data input methods?
- Data encoding = hard copy (analogue) maps and tables
- Data encoding also known as key coding
- Can be used for attribute data only
- Many 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
What is manual digitizing?
Keyboard entry
Drawing - paint mode, node captured per click
stream mode - modes captured at a set rate per time unit.
What is Heads up digitizing?
What is scanning?
and the advantages and disadvantages.
- Automatic digitizing = Conversion of paper (analogue) map to digital format.
- Results in gridded data or vector
Disadvantage: Need lots of cleaning after scanning is completed; especially removal of non-target features.
Advantage: cost-effective for capturing very large volumes of data
- Quality – depends on resolution of the scanner
What is GPS?
**A system of earth-orbiting satellites that provide location (in lat/long
coordinates or equivalent) 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. **
- Primary vector data.
- Derives spatial coordinates of spatial feature/phenomena using a processes triangulation
- Achieves accurate positional readings through triangulation of signals from at least 3 satellites
- Publicly available GPS Satellite system (NAVSTAR) developed by the US military.
What are GPS segments?
Space segment
Control
User segment
What is space segment?
- Constellation of 24 (upgraded to ~32) Satellites or “Space Vehicles” (SV) ➔ Arranged in six equally spaced orbital planes
- Orbits @ Altitude of 20 183km.
Satellite that sends a radiowave /pulse that is sent to the control segment.
What is the control?
Monitor stations
that track and monitor, update and synchronise SV’s.
What is the user segment?
GPS receivers
Apps - google maps
Software
What are the basic processes of capturing the approximate location? (never exactly)
Triangulation:
- Receiver calculates distance to available satellites using the travel time of radio signals
- Through triangulation, receiver identifies it’s own position on the earth’s surface
- We’re using the word “triangulation” very loosely here because it’s a word most people can understand, but purists would not call what GPS does “triangulation” because no angles are involved. It’s really “trilateration.”
Examples of GNSS
- GLONASS (Global Navigation Satellite System) – Russian.
- Galileo positioning system – EU & ESA.
- COMPASS navigation system (aka Beidou-2) – China.
- IRNSS (Indian Regional Navigational Satellite System) - India.
GPS accuracy
Consumer grade - 15-30m eg. garmin, magellan.
Mapping grade - 1-5m e.g GeoExplorer, CMT Mach III
Survey grade - sub-m-cm e.g. Trimble post-processing or real- time.
What are GPS error sources?
- Since your GPS unit is a radio receiver, unintended signals can be a source of error in calculating a position
- Satellite-based uncertainties
- Clock (orbital) error = 1 m
- Positional error (ephemeris)
- Errors due to signal propagation (scrambled signal)
- Atmospheric delay error = up to 11 m
- Multipath error (buildings) = 0.5 m
- Received-based uncertainties
- Clock bias, antenna centre, noise
- User errors
- Receiver software or hardware failure
Position of dilution of precision (3D)
How do I know how much error there is?
Error measurements that your GPS will show you:
* 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).
What is remote sensing?
- Deriving information about the earth’s surface using images that acquired from satellites or high flying aircrafts
- Information is acquired about an object/area/phenomenon without being in physical contact with it
- Images are acquired using electromagnetic radiation in one or more regions of the electromagnetic spectrum.
- Satellites provide continuing program of data acquisition for entire planet.
- Temporal resolution of weeks to several hours!
- The temporal resolution of a satellite is defined as the amount of time needed to revisit and acquire data for the exact same location. The temporal resolution is high when the revisiting delay is low and vice-versa
- Increasingly rapid integration of RS data & analysis into GIS
- Primary data source for input into GIS
- Originally RS software now has integrated GIS capabilities, like
- IDRISI
- ERDAS Imagine
- ENVI
- Given extreme importance of RS datasets as input to GIS… critical for GIS technicians/ professionals/ analysts to gain understanding of RS systems.
What are remote sesning satellites?
EOS (Earth Observing system)
* ASTER (advanced spaceborne thermal emission & reflectance
radiometer) & MODIS (moderate resolution imaging spectroradiometer)
* ASTER designed for scientific/research applications (grain: 15-90m)
* Spatial MODIS designed for global change research (grain: 250-500m)
Landsat
* operation since 1972, currently Landsat7, 8 (ETM+)
* For global change research
* Resolution 30m
* In Sentinel -1,-2,-3
* Part of a constellation of satellites (Copernicus Programme)
* Used for many applications e.g. change detection , natural disaster management
SPOT5/ SPOT6
* Commercial applications
* Resolution 1.5-5m
Quickbird and Ikonos
* High spatial resolution (less than m)
Remote sensing aerial photos?
- Cameras mounted in light aircraft
Vertical photographs
* Taken with camera pointing straight down
* Use stereoscope to create 3-D view…
Oblique photographs
* taken at angle to ground easier to interpret than vertical BUT difficult to make measurements for mapping.
What is data validation?
- Attribute accuracy
- Logical consistency
- Completeness
What is 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
What is Logical consistency?
▪ Does the data make sense ?
▪ Do water pipelines connect at nodes? (logical interpretation and validation of spatial data).
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Introduction to GIS14
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Data abstraction and Spatial Data models17
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Co-ordinate systems17
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Map projections16
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Cartegraphic Representation42
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Topology13
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Spatial data structures27
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Data Management: Attribute Data39
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Data Standards: Scale, Accuracy, Precision24
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Data input25
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Metadata13
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Spatial data management28
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Attribute Data Management16
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Data exploration and classification38
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Spatial & Attribute Queries13
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Spatial analysis: Vecotr analysis17
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Spatial analysis: Raster features23
