Avoiding the Common GPS Pitfalls

1

• Avoiding the Common GPS Pitfalls
• Proper Field Techniques
• Tom Rogers
• trogers_at_seilerinst.com
• Scott Smith
• ssmith_at_seilerinst.com

2
Equipment Setup

• Current OS and GPS Firmware installed.
• Current Date and Time on the data collector.
• Data collector is fully charged.
• Turn off WiFi and Bluetooth to conserve battery.
• Automobile charger and serial clip for charging.
• Factory reset the unit during a off/slow period.

3
Current 2005 Geo OS and GPS FWStart/Settings/Syst
em/System Information
4
Current 2008 Geo OS and GPS FW
Start/Settings/System/System Information
5
Logging Settings in TS 3.3

• 1 second interval is default.
• Auto Pause Count for point/vertex.
• Horizontal/Vertical and In the Field/Post
  Processed accuracy settings.
• Post Processing Distance from Base.
• Accuracy based logging.
• Estimated Accuracy icon on the status bar (click
  on it to display).

6
Open conditions Sub-foot to Decimeter
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Multipath

• Occasionally multipath errors
• still occur.
• Estimated error doesnt always
• include multipath.
• Zephyr has better multipath
• rejection (Stealth technology).

8
Best Practices In the field

• Minimizing multipath
• Multipath is like TV ghosting, caused by
  reflected signals adding errors to the distance
  measured to the satellite.

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Best Practices In the field

• Collect good GPS measurements between features
• Treat the measurements before and after the
  feature as if they are just as important as those
  logged at the feature itself because they are!
• Keep the GeoExplorer unit high.
• Face antenna to the Southern Sky when using unit
  without external antenna.
• Hold the antenna horizontal while walking.
• Dont drop the antenna to your side (or it will
  pick up multipath).

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Best Practices In the field (cont)

• Collect positions for at least 30 seconds at each
  point feature
• In theory, you dont have to collect too many
  positions in a point feature, once the accuracy
  indicator readings are good enough.
• But if you collect for about 30 seconds, you
  increase the chance of acquiring more satellites
  or cancelling out short-term multipath errors.
• If the accuracy indicator readings are poor, you
  should log more positions.
• This will typically be necessary in a hostile GPS
  environment.

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Best Practices In the field (cont)

• Antenna position
• Can you really hold the antenna still directly
  over the target?
• Use a pole, bracket, and plum line or bubble.

12
Log Now or Later

• Log Now start
• a feature and
• simultaneously
• start collecting
• GPS position.
• Log Later start
• a feature, enter
• attributes and start
• collecting GPS
• positions later. Use this for lines and
  areas too. Also best for points if not using an
  external antenna.

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Best Practices In the field

• Pause logging before entering attribute
  information
• Allows you to concentrate on positioning
• Holding the antenna over the location to be
  measured.
• Holding the antenna horizontal with a good view
  of the sky.
• It is not easy to do this while concentrating on
  entering attribute information.

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Best Practices In the field

• Avoid blocking satellites with your own body by
  using an external antenna and pole.
• Monitor your In The Field Accuracy.
• The majority of satellites are between you and
  the equator.

Denver, USA
Frankfurt, Germany
Christchurch, New Zealand
15
In Field Planning

• Planning skyplot
• View future satellite constellation, including
  trajectories and trails.
• Use time in the field more efficiently
• Increase productivity

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Best Practices In the field

• Buildings or other obstructions
• Wait for stable carrier lock.
• Avoid blocking satellites.
• In difficult locations, move the antenna slowly.
• Use the Planning software to see if better DOP
  will be available later.
• Use offset measurements or Laser Rangefinder.

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DistanceBearing

• - See Distance and
• Bearing diagram

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Distance - Bearing
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Under trees Not always subfoot

• Depends on
• Number of satellites tracking carrier (5 plus).
• Carrier lock duration (2 minutes).
• Distance to base station (50 miles).

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Best Practices In the field

• Trees and canopy
• Use an external antenna on a pole.
• Wait for a stable lock before entering canopy
  environments.
• Use the Planning software to see if better DOP
  will be available later.
• Use offset measurements or Laser Rangefinder.

21
Questions?

• http//www.seilerinst.com/mapping/support.asp
• Tom Rogers
• trogers_at_seilerinst.com
• Scott Smith
• ssmith_at_seilerinst.com