Lect 06 Coordinate System Flashcards
What is a Coordinate System?
A standardized method for location codes (latitude/longitude, x/y axis)
How many types of coordinate systems are in GIS?
- Geographic Coordinate System
- Projected Coordinate System
In a coordinate system:
Easting is x direction (Longitude)
Northing is Y direction (latitude)
What happend if two map layers have different coordinate systems?
Problem: They will not align spatially
Solution: Standardize their coordinate systems
What is a geographic coordinate system?
*Locates features on earth using longitude and latitude.
* Measures angles in degree, degree-minutes-seconds (DMS), decimal degrees (DD), or radiands (rad)
Geographic Coordinate System
GCS range:
Latitude: 90º S to 90ªN
Longitude: 180ºW to 180 ºE
What are parallels:
Lines of constant latitude
What is a Meridian?
Lines of constant longitude
Difference between sphere and ellipsoid
The earth is often modeled as a sphere for map projections but it´s actually an ellipsoid with a larger equatorial radius
What is ellipsoidal?
A smooth average of the geoid, approximating earth´s shape that can differ from local sea level
What is Geoid?
*Surface of constant gravity
*Approximates local sea level with more variation than ellipsoid
What is ellipsoid height?
Height on an ideal surface used GPS and satellite imagery (NAD83)
What is Orthometric height?
Elevation above the geoid, used in surveying and land management eg., NAVD88.
What are the three commonly used ellipsoids?
- Clarke 1866
- GRS80 (Geodetic Reference System 1980)
- WGS84 (World Geodetic System 1984)
Why not the same ellipsoid?
Historically, geodetic surveys were limited by water bodies and relied on optical telescopes, constrained by Earth´s curvature.
Each continent had its tailored ellipsoidal parameters due to these isolated surveys.
Recently, satellite, laser, and timing signal data allow for precise global position measurements across continents and oceans.
What is Datum?
A mathematical earth model, foundational for geographic coordinates.
Defines Earth´s size, shape, origin, and mapping orientation.
Determined by an ellipse and rotation axis, it comprises:
1. Ellipsoid parameters and origin
2. Points and lines set
What are the two types of Datum?
Geocentric datum (x, y)
Based on ellipsoid (e.g., NAD8, WGS84)
cf. NAD27 is a local datum
Vertical Datum
References the geoid (e.g., NAVD88)
What are benchmarks?
Points surveyed during datum development and marked with monuments
What is vertical datum?
Horizontal control networks
* Provide positional data related to a mathematical surface (e.g ellipsoid).
*Vertical Control Networks
* This elevation, called orthometric height, is determined by spirit leveling with gravity measurements.
What are the main horizontal datums?
1) North Americam Datum 1927 (NAD27)
2) North American Datum 1983 (NAD83)
What is Map Projection
Converting a three dimensional surface (like earth) to a two dimensional (flat) surface
What are some of the advantages of Map Projection?
- Allows for 2D paper and digital maps over globes.
- Uses plane coordinates instead of longitude & latitude.
- Offers straightforwraddistance and area measuraments compared to GCS.
Distorition in Map Projection
1) Map projection always have errors and spatial distortions.
2) The right projection preserves properties and reduces distortions in targeted areas.
Four properties for preservation
- Conformal (Shape)
- Equivalent (Area)
- Equidistant (Distance)
- Azimuthal (Direction)
Azimuthal
For global views
Cylindrical (Transverse Mercator)
For North-South Areas
Conic (Albers, Lambert Conformal)
For East-West areas
State Plane Coordinate System
Based on state and county geography
Used for legal boundaries in civilian systems
Ensures top local measurements accuracy
120 Zones, shaped by state and zone contours
Global Correction Service (IGS)
Operates a global network of over 512 tracking stations.
A voluntary group of more than 350 agencies producing precise GPS/GLONASS products.
Committed to top-quality data, setting GNSS standards.
RINEX (Receiver Independent Exchange Data Format) - IGS
- A standardized format by IGS (Used for archiving and online access at the global data center)
- With accuracy level depending on availabilty
Regional Correction Services
- CORS (Continuously Operating Reference Station)
1. United States.
2. Over 2,000 continuously operating stations.
3. Providing GPS measurements for 3D positioning.
Regional Correction Services
CACS (Canadian Active Control System)
Canada
Operates continuously.
With over 135 stations.
Offers real-time positioning accuracy within one meter and post-processing accuracy within three centimeters.
Both CORS and CACS stations are closer to the reference stations than the IGS stations.
DGPS Radio Beacon System
Marine radio beacons at lighthouses
and coastal locations are electronic
navigation aids operating in the low-to-
medium frequency band (283.5-325
kHz).
DGPS Radio Beacon System
A reference station (RS) generates real-
time DGPS corrections in the RTCM (Radio
Technical Commission for Maritime
Services) format.
With an integrity-monitoring (IM) unit
overseeing its performance.
Free for all users.
RTCM (Radio Technical Commission for Maritime Services) Format
Coastal networks of reference stations.
Continuously transmits real-time DGPS corrections with the
RTCM format.
Enhancing marine navigation safety.
A beacon receiver connected to a GPS receiver that accepts RTCM corrections is needed to use this service.
RTCM (Radio Technical Commission for Maritime Services) Format
GPS receivers that accept RTCM corrections are known as
differential-ready GPS receivers.
Offering accuracy from sub-meter to a few meters.
Free for the general public.
Radio Beacon Receiver
Combination of Beacon/GPS Receiver:
Micro-Trak T100
Single Unit
DGPS Radio Beacon Receiver:
Trimble beacon-on-a-belt (bob)
DGPS Radio Beacon System
These receivers pick up the transmitted DGPS corrections and come in single- or dual-channel options, with dual-channel being more reliable but pricier.
The official range is 150 miles, as per the Coast Guard.
Coverage depends on factors like transmitter power output, atmospheric
noise, receiver sensitivity, and propagation characteristics, which are better
over water than inland areas.
Beacon locations are strategically selected for overlapping coverage to
enhance accuracy.
DGPS Radio Beacon System
- Differential corrections come from the NAD 83 position of the reference station (REFSTA) antenna, so DGPS positions should align with the NAD 83 coordinate system.
WADGPS (Wide Area DGPS) Systems
- A satellite-based differential correction service
- Using widely separated reference stations
- Providing sub-meter accuracy
- Utilizing the RTCA (Radio Technical Commision for Aeronautics) format for aviation telecommunications
WADGPS (Wide Area DGPS) Systems
Real-time DGPS with a single reference station faces the challenge of declining accuracy:
(1) Users moves farther from the reference station.
(2) With the highest accuracy limited to a small area around the reference station.
Fix: WADGPS is employed.
WADGPS Systems
Steps outlining how the WADGPS system
operates:
1.Reference stations
Gather the GPS data and transmit it to the
master station.
2.Master station
Analyzes correction data and uploads it to a
geostationary satellite.
3.Geostationary satellites
Transmit the data to a local GPS receiver.
4.GPS receiver
Applies the necessary corrections.
WADGPS Systems
WADGPS includes four satellite-based augmentation systems:
1. WAAS (Wide Area Augmentation System) in North America
2. EGNOS (European Geostationary Navigation Overlay System)
in Europe
3. MSAS (Multi-Functional Satellite Augmentation System) in Asia
– Japan
4. GAGAN (GPS and GEO Augmented Navigation) in Asia - India
Geostationary Satellite
A geostationary satellite, or GOES (Geostationary Operational Environmental Satellite).
Orbits the earth at the same rate as its rotation.
Maintaining a fixed positioning over the equator.
Geostationary Satellite
Often called TV satellite.
Completes a 24-hour orbit that matches the Earth’s rotation.
In contrast, GPS satellites orbit twice a day, finishing their orbit in 12 hours.
Satellite Orbit
- Low Earth Orbit: Sun - Synchronous
- Medium Earth Orbit: Semi-synchronous
- High earth orbit: geo-synch
WAAs VS. LAAS
Both WAAS and LAAS are GPS augmentation systems that enhance accuracy, availability, and integrity.
WAAS (wide-area augmentation systems) is satellite-based.
LAAS (local-area augmentation systems) is ground-based.
WAAS (Wide - Area Augmentation Systems)
A Space Base Augmentation System (SBAS) supported by the FAA and DOT.
Specifically implementing WADGPS.
Enchains GPS signal accuracy.
Initially designed for civil aviation, its coverage now extends to inland and
offshore areas, making it suitable for land and marine applications.
Continental DGPS systems are limited to North America due to no ground
reference stations elsewhere
WAAS (Wide - Area Augmentation Systems)
Pros: WAAS requires no additional receiving equipment and offers broader coverage, including inland and offshore areas, compared to land-based DGPS.
Cons: Signal reception can be hindered by obstructions like trees or mountains due to the satellite positions over the equator.
WAAS - How does it work?
- Ground stations: Collect GPS data and send it to the master station.
- Master station: Analyzes correction data and uploads it to a geostationary satellite
- Geostationary satellites: Transmits data to a local GPS receiver.
- GPS receiver: Apply the appropriate correction.
WAAS - Accuracy
- Differential corrections provided by WAAS increases the accuracy of C/A signals.
LAAS (Local Area Augmentation System)
- Achieves higher accuracy through local-area base stations.
- Operates on a smaller scale
- Reference receivers near runways provide significantly more accurate correction data to incoming planes
LAAS (Local Area Augmentation System)
Are located near airports
Broadcast correction messages within a limited range of 20-30 miles.
Uses a VHF (very high frequency) radio data link
LAAS accuracy is less than one meter
LAAS Air Navigation
- Reference receivers, positioned close together,
collect measurement errors. - They broadcast correction messages via a VHF
(Very High Frequency) radio data link
Why integration with GPS?
GPS applications enhance accuracy and global availability.
However, they face signal obstruction in some scenarios.
e.g., urban canyons and deep open-pit mining
Solution: Integration of GPS with other positioning systems
GPS Integration
To overcome signal obstructions, GPS is integrated with other
systems like:
1. GIS
2. LORAN (Long-Range Navigation)
3. LRF (Laser Range Finder)
4. Dead Reckoning
5. INS (Inertial Navigation System)
6. Pseudolite
7. Cellular systems
GPS/GIS Integration
GPS efficiently collects precise GIS field data in digital format, in real-time or post-processed.
GPS-GIS integration serves various industries.
e.g., utilities, forestry, agriculture, and public safety
GPS/GIS Integration: Mobile GIS
Trimble Juno SB
* Offers a positioning
accuracy of 2 to 5
meters in real time and
1 to 3 meters when
post-processed.
GPS/Loran-C Integration
LORAN (Long-Range Navigation) is a terrestrial radio-navigation system with master and secondary stations
The LORAN system was phased out in 2010
GPS/LRF Integration
GPS signals under tree covers are challenging.
Solution: LRFs (Laser Range Finders)
Post-processing is needed to combine the data.
GPS/LRF Integration
When:
In inaccessible under tree canopies
In areas with poor GPS visibility
How:
Set up a GPS antenna in a clear-sky open area
Use an LRF to determine distances and azimuth to objects
Offset function: Records feature offsets while staying in one
spot
GPS/LRF Integration
Applications
Utility industry:
e.g., electric, gas, and water utility
companies
Forestry and natural resource
e.g., fire prevention, control/harvesting
operations, insect infestation, boundary
determination, and aerial spraying
10.14 Cadastral surveying
e.g., establishing property corners,
boundaries, and areas of land parcels.
GPS/DR Integration
The Dead Reckoning (DR) system supplements GPS in areas with poor signal reception.
It estimates position from a known point using factors (e.g.,
direction, speed, time, and distance).
DR is cost-effective, using:
Odometer sensors (measure vehicle distance/speed)
Vibration gyroscopes (measure vehicle direction with low-
cost angular rate sensors)
GPS/DR Integration
When:
In urban areas where, GPS quality is vital, but obstructed by
buildings
How:
Use the odometer for distance and gyroscope for direction
Limitations:
GPS unavailable in obstructed areas
DR drifts over time, causing positional errors.
GPS/DR Integration
In vehicle location & navigation:
GPS helps calibrate DR, which serves as the main system
during GPS failures.
GPS/DR integration provides better estimates for positioning
and direction.
However, it is NOT ideal for high-accuracy applications.
GPS/INS Integration
Mining and airborne mapping need high-accuracy positioning in
obstructed or dynamic conditions due to GPS limitations.
Solution: GPS/INS integration
Inertial Navigation System (INS) use inertial sensors (IMU) to
determine vehicle position, velocity, and orientation.
INS is environment-independent, jamming-resistant, and more
accurate than dead reckoning (DR).
GPS/INS Integration
An INS includes accelerometers (measuring acceleration) and
gyroscopes (measuring rotational velocity).
Once initialized, it provides 3-D position and velocity
autonomously.
Drawback: Drift over time, causing unbounded errors if unaided.
