LAB 1
METHODS FOR LOCATING
YOUR FIELD DATA IN
GEOGRAPHIC SPACE
Geog 315 / ENSP 428
Lab 1 Schedule
Introduction to bio-physical field data collection (8:00-8:20am)
Locating your data on the earth: NAVSTAR Global Positioning System (8:20-9:20am)
-- 15-min Break --
Quiz (9:35-10:00am)
Measuring distance and azimuth(10:00-10:30am)
--15-min Break –
Planning the “field campaign” (10:45am-11:10am)
Introduction to Trimble Juno GPS units and Impulse laser rangefinder(11:10am-11:40pm)
Lab Objectives
Understand the spatial dimension of field data, and the costs and benefits of collecting it
Understand how the Global Positions System (GPS) works, its advantages, and its limitations
Understand how distance between points can be surveyed with laser rangefinders, and know when this technology is appropriate to use
Learn how to effectively plan for a field campaign to increase sampling efficiency and spatial and attribute data accuracy
Exposure to GPS units and laser rangefinders used in Lab 2
Collecting bio-physical field data
La Selva Biological Field Station
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1000 0 1000 Meters
Soils / SuelosOld alluvium / Aluvion viejoRecent alluvium / Aluvion recienteResidual
Stream-associated / Suelo de quebradasSwamp / Pantano
# Species present / Presencia del especies
N
Dipteryx panamensis
Elephant herd home range
http://www.save-the-elephants.org
Spatial information
• Most spatial information of interest is geographic –
can be placed on the earth
• Spatial information links a place with a property of that
place– The temperature is 40° C at
Latitude: 38° N, Longitude: 122° W
• Could also be at a specific time– The temperature is 40° C at Latitude: 38° N, Longitude: 122° W at
02/13/2009, 8 am
• Properties are variables that we measure – sensed
with our body or instruments
• Can be quantitative or qualitative
• The potential number of geographic places and their
properties is vast…
Some bio-physical variables of interest
Biological – plants, animals, fungi, etc.
Composition – presence/absence, abundance of species or groups, communities
Vegetation structure – biomass, height, diameter, percent cover, leaf area index
Physical environment variables
Moisture, temperature, light
Geological composition
Chemistry of soil, water or air (e.g., nutrients, pollution)
Geomorphology – shape of the land, including slope, aspect, elevation
Disturbance and threats
Natural: fire, wind-throw, pest attack, species invasion
Anthropogenic: deforestation, poaching, grazing
Sampling geographic information
Type
Single points
Transects along a line
Plots
Rapid assessment visit
Scale
Space – boundary area; geographic area or length of samples; distribution of samples
Time -- return interval. Once every year? Need to come back to location? Need to leave a monument?
Considerations
Preliminary assessment or long-term monitoring
Time
Money
Access
Global Navigation Satellite Systems
(GNSS)
Locating your data on the earth
Global Navigation Satellite Systems (GNSS)
► Most common approach to surveying locations is using Global Navigation Satellite Systems (GNSS)
► It uses range measurements based on radio signals from satellites
► Systems developed by USA, Russia, European community and China
► Developed, owned and maintained by the U.S. Department of Defense (US$400 million per year to maintain)
► Accurately determines horizontal location, elevation and speed
Almost anywhere on earth
day or night
any weather
► Free for public use!
NAVSTAR
Global Positioning System (“GPS”)
GPS segments
GPS satellite segment
32
http://science.nasa.gov/Realtime/jtrack/3d/JTrack3D.html
GPS user segment
GPS General Information
GPS satellites broadcast three different types of data using radio waves
1. Almanac data
- system health and rough orbits of all GPS satellites; tells receiver which satellites to “listen” for
2. Ephemeris data for the broadcasting satellite;
- allows a GPS receiver to accurately calculate the position of the broadcasting satellite
- satellite health, clock corrections, etc.
3. Coded signals
- Coarse Acquisition code, or C/A, and the Precise code, or P-code (C/A code used mainly in civilian applications)
Range = speed of light x travel time
Range = c(t1 – t0)
(c =299,792,458 meters per second)
t0
t1
A single satellite
range measurement
GPS code receiver
Assume that satellite and receiver are generating the same pseudo-random code at exactly the same time
Measurements to multiple satellites determines position
Sources of Error
Several factors can result in erroneous location
determination with GNSS (or GPS)
Source Range Error (m)
Satellite clock error 1
Satellite position error 1
Atmospheric & Ionospheric effects 4
Receiver error 1.5
Total ~7.5
Positional uncertainty (1)
Positional uncertainty (2)
Leads to positional
uncertainty…
Ionosphere
http://www.aiub.unibe.ch/ionospherehttp://apollo.lsc.vsc.edu/classes/met130/notes/chapter1/ion.html
• Electrified region within the upper atmosphere• Can reflect, deflect and scatter radio waves – increase range
Source:http://www.garmin.com/aboutGPS/
Multipath signal error
Positional Dilution of Precision (PDOP)
Obstructions to satellite signals
Telescoping pole
Adjusting PDOP thresholds
Typically want PDOP < 6
Types of receivers (1)
Many types of receivers on the market – vary in price, features and performance
Most now are multi-channel (12) – can track up to 12 satellites
Ability to average points
Ability to change projection and datum
Display screen for features and maps
Memory to hold features and properties (attributes)
Ability to download data
Battery life
Differential corrections (more on this coming)
Antennas that reduce error
Types of receivers (2)
Recreational receivers - $200 to $1000
Mapping grade - $1000 - $10,000
Set PDOP, satellite elevation and signal-to-noise filters
Target satellites
Point averaging
Differential corrections
Data dictionary and data download
High-end survey grade
Better antennas
Can achieve centimeter accuracy
Differential correction
For each satellite, the roving (receiver) range is corrected by the observed range error at base station
Differential correction
Two common
types of
differential
GPS
Best performance
if base station
within 180 miles,
300 km
CORS - Continuously Operating Reference Stations
National Geodetic Survey (NGS), an office of NOAA's National Ocean Service, coordinates a network of Continuously Operating Reference Stations (CORS) –base stations
Each CORS site provides carrier phase and code range measurements for differential correction
GNSS - GPS and GLONASS supported
CORS data are available at their original sampling rate for 30 days, after that at reduced sampling rate
http://www.ngs.noaa.gov/CORS/
CORS
http://www.ngs.noaa.gov/CORS/GoogleMap/
Measuring distance and azimuth angles
Distance and azimuth measurements
Offset points: We can locate a fixed point with a GPS (GNSS) receiver and then calculate the horizontal position of other points relative to the GPS point with distance and azimuth (bearing) angle measurements
The GPS point in this context is called a control point
Azimuth: the clockwise angle from north, e.g. 45o
(northeast), 180o (south) – i.e., bearing
The accuracy of this technique depends on accuracy of instruments used to measure
Geographic position (e.g., GPS)
Angle (e.g., analog or digital compass)
Distance (e.g., tape measure, laser rangefinder)
Compass
Measures azimuth angles
0º to 360º
Magnetic declination
Earth has a geographic north and south pole – axis
upon which the planet spins
Earth is like a big magnet; liquid iron-nickel core
creates magnetic field
Compass needles point in the direction of the
magnetic field lines – “North” on a compass, or 0º is
magnetic north, not geographic north
Magnetic declination - angle between the compass
pointing direction and geographic north, or true
north
http://www.ngdc.noaa.gov/geomagmodels/struts/calcDeclination
Distance
Tape measures
Laser rangefinder
Maximum
Range575 m
Accuracy 3 - 5 cm typical
Inclinometer
Range± 90 degrees
Inclinometer
Accuracy± 0.1 deg. typical
Horizontal distance
Want horizontal distance, or planimetric distance --
may need to slope correct
A
B
hd = horizontal distance
Θ = inclination angle
Cos (θ) = hd/sd (adjacent/ hypotenuse)
hd = sd * Cos (θ)
Θ can be measured with a inclinometer
…or laser rangefinder inclinometer
Vertical height
Some applications require measurement of height
Same concepts apply…
hd = horizontal distanceA
B
θ1
Tan(θ) = vh/hd (opposite/adjacent)
vh1 = hd * Tan(θ1)
vh1 = vertical height
vh2= vertical heightθ2
C
vhtotal = vh1 + vh2
Measuring tree height, La Selva, Costa Rica
Using the Laser Tech Impulse
Mapping a point
Forest
GPS control point &laser rangefinder
θ
x
y
Sin (θ) = x/hd (opposite/ hypotenuse)
x = Sin (θ) * hd
Cos (θ) = y/hd (adjacent/ hypotenuse)
y= Cos (θ) * hd
Trunkx = GPSx + x
Trunky = GPSy + y
0˚ N
Field Plot
Mapping a polygon (1)
Collecting corner x,y positions with a GPS receiver
GPS positional error
Field Plot
Mapping a polygon (2)
Collecting corner x,y positions with a GPS receiver
and differential corrections (DGPS)
DGPS positional error
Field Plot
Mapping a polygon (3)
Collecting corner x,y positions with DGPS and laser
rangefinder
Control pointDGPS positional error
Laser rangefinderpositional error
Closure?
Planning the “field campaign”
Project fundamentals
Define research question and goals
Consider a spatial perspective in questions
Ask how spatial data and sampling scheme help
answer this
Familiarize yourself with study area
Logistics of getting there
What type of obstacles – canopy cover, mountains
Permissions for access, other cultural issues
Project fundamentals (cont)
Resolution and accuracy needs
Spatial and temporal scale
Type of equipment: recreational or mapping-grade
GPS?
Tape measure or laser rangefinder?
Data collection methodology
Points, lines or areas
Coordinate system and projection
How data collected? Who?
How data stored – data dictionary, on a paper form
Long-term field plots
Field data
form
Data dictionary
A data dictionary is a "shopping list" of the features and their attributes that you want to map in the field
You create the data dictionary with the GPS vendor’s software (e.g., Pathfinder Office) prior to going into the field
You then upload the data dictionary to your GPS receiver
Once in the field, the data dictionary prompts you for information for each spatial feature (e.g., point, polygon) measured
Provides a standard format for data entry
Saves time!
Helps prevent input errors!
Mission planning software
GPS (GNSS) vendors generally provide mission planning software with receiver
Free software is from Trimble
Almanac information on satellites is available from Trimble, http://www.trimble.com/gpsdataresources.shtml
http://www.trimble.com/planningsoftware.shtml
Data dictionary
GPS satellites - 02/26/2010
Sky plot
Rohnert Park, CA 02/26/2010
Number of satellites – Santa Rosa, CA
Rohnert Park, CA 02/26/2010
PDOP – Santa Rosa, CA
Rohnert Park, CA 02/26/2010
Sonoma State Campus – June 2007
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