Transcript GPS Best Practices -- Field Chris Wayne (CRLA) channeled through Tim Smith

GPS Best Practices -- Field
Chris Wayne (CRLA) channeled
through Tim Smith
April 1, 2008
Session Overview
• Discuss best practices for field techniques
when conducting GPS-based missions
• Field recon, antenna positioning, pre-field
preparation and other important stuff
• techniques for maximizing accuracy and
completeness of field-collected data
• “software generic” but examples use
Garmin and Trimble workflows.
Review of pre-planning session:
• “Now you’re ready… right?”
• Verify everything: GPS settings, batteries,
personal gear, gas in the rig
• Test it all before you leave the lab,
especially the GPS
Field Recon – Know before you go
• Be realistic about what you can
accomplish in 1 day
• Round trip? Go to your most distant point
then map your way back
• If you don’t finish, you have less distance
to cover when you return
• Pay attention to features that will become
attributes
Safety First!
• Water, food, communication
• Plan left with responsible party: departure
time & place, return time & place
• Map and compass and competency with
them.
• DO NOT TRUST YOUR LIFE OR YOUR
MISSION TO A GPS UNIT!
– (Would you trust life or mission to a double-A
battery?????)
Getting geared up
• “10 essentials”
for hiking
• Camera
• Rite-in-rain
Notebook
• Other field forms
on R-in-R paper
• GPS Quick Guide
• FOOD!!!!!!
Define and Review Mission Goals
• Positional Requirements:
– Mapping into GIS: Points, lines, polygons
– Relocation by GPS and/or map and compass
• Attribute requirements
– Just a name and/or ID?
– GIS-quality attributes?
• Project requirements
– What must be done by when?
– Fast, accurate, cheap: pick 2
Attribute Collection
• Minimum: Unique ID (name or number) of a
position or feature
• Ideal: Detailed, multi-field descriptions of
positions or features that meet pre-defined
standards
• Reality: A bit of both
• Geodatabases and data dictionaries help us
reach the ideal
• ALWAYS include an “Other as Encountered”
option in menus, AND a “Notes” field
Ways to collect attributes
• Rec Grade (Garmins, Magellans):
Waypoint metadata forms, available on
NPS GIS Site
– Requires careful manual notekeeping
• Data Dictionary: Built in Trimble Pathfinder
Office, deployed with Terrasync
– Will require conversion to use in ArcGIS
• Geodatabase: Built in ESRI ArcCatalog,
deployed in field with ArcPad
Workflow Examples
• Trimble: Mapping-grade units
– More accurate, correctable, steep learning
curve
• Garmin: recreational-grade units
– Less accurate, not correctable, learn on the
fly
• Table to compare and contrast
Trimble - ArcPad Workflow
Example
• Check out data (or data schema) from
Geodatabase using ArcMap
• Upload data to Trimble unit
• Collect field data with ArcPad
– GPSCorrect optional but required for post-processing
• Download field data from Trimble unit
• Post-process field data (either in Pathfinder
Office or ArcMap w/GPS Analyst extension)
– Only possible if you used GPSCorrect while collecting
• Check corrected field data back into
Geodatabase using ArcMap
Trimble - Terrasync Workflow
Example
•
•
•
•
•
•
Build data dictionary in Pathfinder Office
Upload data to Trimble unit
Collect field data with Terrasync (.ssf files)
Download field data from Trimble unit
Post-process field data in Pathfinder Office
Convert corrected field data back into
Geodatabase using ArcMap
Garmin – ArcMap Workflow
Example
• Configure GPS unit: coordinate system, datum, etc. *
• Upload background maps to Garmin unit using
MapSource *
• Upload tracks and waypoints (if needed) using
DNRGarmin or ArcMap
• Collect field data : tracks and/or waypoints
• Turn off tracklog before turning off GPS! Why..? See
upcoming slide!
• Download tracks and/or waypoints from Garmin using
DNRGarmin
• Export field data (tracks and/or waypoints) to ArcMap
using DNRGarmin
•
* These tasks normally have to be done only once, unless the Garmin runs out of power or the settings get changed. Always verify
before you go into the field!
Garmin vs. Trimble
Garmin
Trimble
Hardware
70Csx, 60Csx, etc
GeoGT, GeoXH (ProXRS)
Background Data Source
Garmin MapSource
GIS Data from ArcMap
Field Software
Embedded menus and pages
ArcPad -- Windows-based or
Terrasync
Accuracy
2-30 meters
0.5 - 5 meters
GIS Conversion Software
DNR Garmin to ArcMap
ArcMap or Pathfinder Office
Managing Garmin Tracklogs
• Turn off tracklog before turning off GPS!
• Otherwise, your track will connect the last point
collected on your previous mission to the first
point collected on your new mission
• Contrast to ArcPad or Terrasync, where you
must explicitly save each feature before making
a new one
• Mapping is a secondary design consideration for
Garmins, navigation and re-location are primary
Differential (“real time”) correction
• 3 options:
– WAAS: Wide-Area Augmentation System
satellites
– NDGPS: National Differential GPS Beacons
– Onstar Satellite Subscription
• Each must be separately enabled on each
unit
• Not all GPS units support all options
WAAS and Real-time Correction
• Always turn on WAAS
– Works with rec-grade and mapping grade units
– Remember it is only line-of-sight
• NDGPS is active (for now…? Thanks Tim!)
– Most units (e.g..: Trimble) will require additional
hardware to receive: Trimble BoB, Thales Mobile
Mapper
– Won’t work with rec-grade units
• A supplement to, not a replacement for, Postprocessing
Quantity Over Quality?
• Better to get a bad position than no
position?
• Trimble doesn’t think so: many units won’t
allow a 2d position
• You may disagree– dumb down the
minimum quality of positions to increase
quantity (1 is better than 0)
Quantity Over Quality?
• Settings are dynamic—adjust them
according to conditions
• Few options on recreational grade e.g.:
Garmins
• Many options on mapping grade e.g.:
Trimbles
• Personal experience: Garmins are less
accurate but get much better reception
under canopy, in cars, even indoors
How to get better reception in
poor conditions
• You will sacrifice accuracy for productivity!
• Bump up maximum allowable PDOP (lower =
better quality position)
• Allow 2d (no elevation, x-y only)
– Trimble units won’t let you do take 2d
• Change minimum angle: lower angle above
horizon  higher PDOP but more satellites
• Use the pre-plan utility in GPS Analyst or PFO
– Tells you estimated PDOP, available SVs for your
location at any given time
Reading your constellation: where
are the SVs?
• SV = “Space Vehicle” =
Satellites
• Arrangement of SVs in
sky = constellation
• Best constellation:
many scattered across
the sky
• Farther from “bullseye”
= lower on horizon
Canopy and canyon conditions
•
•
•
•
Wood (trunks) is worse than leaves/needles
Rock and buildings worse than wood
Canopy + Canyons = Patience + flexibility
Any antenna helps, especially on a range pole.
Higher the better.
• Even after adjusting PDOP, minimum angle,
raising the antenna…
• Be prepared for walk, stop, wait, wait, walk, stop,
wait, walk… etc.
Listen to your unit!
• Make sure that all warnings are audible,
not just visual
– 2D only! max PDOP exceeded! no SVs! etc.
• You can hear when you lose signal
• Allows you to look up so you don’t walk off
a cliff
• Annoying? Of course.
Antenna Use
• Use an external antenna whenever
possible
• Different grades of antenna: multi-path
rejection, receptivity
• Antenna is the GPS position, not the unit
• Keep it still if collecting a point
• Multiple antennae will interfere with each
other—maintain at least a foot of spacing
• Tip: Duct tape is OK—tape it down
Antennas with Trimbles
• Important to get antenna settings correct
– Antenna type: Zephyr, Hurricane, etc.
– Antenna Height (above ground)
• Set in TerraSync  SetupAntenna
• Or if using ArcPad
– GPSCorrect if available
– PFController if not
– ArcPad takes what you give it w/o question
Serious mapping? Take two
units!
• CRLA example on backcountry trails:
• Mapping-grade unit: Trimble GeoXH
w/Hurricane Antenna running ArcPad,
GPSCorrect
• Backup unit: Garmin GPSMap 76CSx
w/Gilsson MCX Antenna and waypoint
metadata forms
• Don’t waste a day in the woods w/nothing
to show for it!
Navigation Considerations
• To paraphrase Joel: “GPS just messed
everything up.”
• Even a rec-grade unit will have better
accuracy than any map, paper or digital
• Background maps on GPS vary widely in
source and quality
• Even 2005 Orthos are generally based on
1:24k National Map Accuracy Stds = 40’ =
10m
Wrap-Up
• Field work with GPS units is a dynamic
undertaking…
• Not just a walk in the woods with the GPS
• Never trust your life to a GPS unit
• Look up– pay attention to conditions that affect
your GPS reception and your safety (cliff edges,
snags, traffic!)
• Look down– don’t ignore your unit, it may be
telling you something important
• Never trust your life to a GPS unit!