New and Useful Miscellany
Two Scenarios When Differential Correction
Lowers Accuracy
This can happen when the base station is too far away - a long baseline. Know &
record the base station you are using and how far away it is from the field site,
especially if doing real time non-WAAS. TS, ArcPad and similar GPS software
will simply locate the best available base station and use it - regardless of the
specs in your SOP. While it is generally helpful to have this done automatically,
always check on the distance to that software-selected base station and make sure
that baseline distance meets the project requirements.
Some non-CORS base stations do not keep NAD83 epochs up to date. An epoch
is the mean date of the observations used in the regional adjustments to the
datum. Official GPS time is not adjusted and therefore it is offset from UTC by a
number of seconds due to the insertion of ‘leap seconds’. The number then
remains constant until the next leap second occurs. This offset is also given in
the navigation message and your receiver should apply the correction
automatically. Recently GPS time was ahead of UTC* time by 15 seconds.
CORS base stations are similarly updated, but non-CORS bas stations sometimes
fall behind. If you use a non-CORS base station be sure to check on what epoch
it is using.
*UTC = Universal Coordinated Time, the proper name for what is commonly known as Greenwich
Mean Time.
NDGPS (National Differential GPS)
Accuracy is reliable and improving for realtime (broadcast) code level diff corr.
• CG maintains the whole system and has upgraded all coastal stations with Recap (new
receivers and antennas).
• DOT requested funding for interior as well – funded for this year (CG does the actual
• NOTE: if using NDGPS real time diff corr in ArcPad remember that the NDGPS base
stations are all on NAD83, so make sure you are using NAD83 for ArcPad’s datum frame
on the rover. (otherwise ArcPad/Rover assumes incoming base data is in WGS84 and will
shift @1.5 m - in New England)
NDGPS upgrades do Not include High Accuracy broadcasts, which is something we’d
hoped for. HA would be broadcasting carrier level corrections as well as code level
corrections. It would be possible to broadcast L2C and L5 (new code frequencies), carrier
phase corrections, plus corrections for Galileo (European) and Glonoss (Russian/Indian) as
well as regular old code corrections.
A few HA NDGPS stations are broadcasting for testing purposes in Maryland and one coming
soon to Pueblo, CO. Tim is testing the one in Pueblo.
High Accuracy Post Processing
Right now Tim gets 8 cm accuracy with HStar diff corr on a GeoXH. HStar is a
carrier code diff corr (dual frequency carrier code, post processed). This is signal
is available from CORS stations but only for post processing, nothing real time.
International News
Chinese are building Compass GPS for global coverage similar to GPS. Aka
Beidou-2. In a race with Galileo for first use/first rights to specific frequencies
Interplanetary News
The 2012 Solar Max is on the downswing and will continue to have effects on
all things space-based, such as GPS or WAAS. Briefly, the intensity of solar
winds (charged particles from the sun) swings from high to low over a 22 year
cycle. The solar max is the peak of this cycle for its effect on earth. The
unusually large number of charged solar particles inside the earth’s
magnetosphere can cause unusually large atmospheric disruptions which could
affect GPS signals, heighten aurouras (and possibly affect the electrical grid).
This underscores the importance of doing proper differential correction whether
post-processed or real time. Why? Because diff corr mostly serves to correct
atmospheric attenuation of the GPS signals, such as those caused by the solar
ArcGIS Mobile
Set up GPS data collection for people with minimal training
Real time only
Simple preset data collection and field inspection
Centralized management and configuration of projects
Fizzy Calc
• Converts between dd, dm and dms
• Calculates between waypoints and projecting from a waypt from
original one
• A single executable file you can run from a usb drive – no install
What a GPS Mapping Receiver
– Accuracy (How close to Truth)
• Meet National Mapping Accuracy Standards:
– 1:63,360 (32meters)
– 1:24,000 (12 meters)
– 1:5,000 (4 meters)
Able to Average a Point
External Antenna options
Good on Batteries
What a GPS Mapping Receiver
should do
– Export in GIS Friendly Formats
– Capable of Controlling Attributes in the Field
• Data Dictionary (Name, Trail Width, Comments)
– Capable of Datum Transform
– Mission Planning
• Sky Plots
• Planning the day
– Run on standard PC / equipment
Recreational Grade GPS - (e.g. Garmin)
• Features Include:
– Wide Use
– Accuracy:
• Autonomous < 15meters 95% typical
• DGPS ready 3-5 meters 95% typical
– User Friendly
– Many models
• Garmin GPSIII+, GPSMap76, eTrex
– Moving Base Maps (1:100,000 scale)
– Free software designed to allow download and upload of
waypoints, tracks and routes from the Garmin to a PC
Mapping Grade GPS - (e.g. Trimble)
• Features Include:
– Wide Use
– Accuracy:
• Autonomous < 15meters 95% typical
• Post-Process DGPS
– 3-5 meters 95% typical (Geo3)
– 1 meter 95% typical (ProXR)
– Pathfinder Office Software
• Data Dictionary, Planning
– Complex Feature mapping (pt/ln/poly)
- Attributes
- Waypoints
GPS for GIS - Pros and Cons
• Garmin III+
– Pros
Cheap and consistent 15 meters positions - good under canopy
Good “Point tool”
Fun and easy to use
Integration into ArcView - relatively straightforward
Battery usage is great
– Cons
Position accuracy not so trustworthy
Not designed for GIS Mapping - relies on 3rd party software
No Data Dictionary capability
Decent storage of tracks/waypoints (6-10 hours)
GPS for GIS - Pros and Cons
• Trimble GeoExplorer 3
– Pros
1-5 meter mapping tool
Collects for 16 hours
Software is excellent
– Cons
Firmware will be changing?
Under-canopy issues - external antenna recommended
Relatively small storage of tracks/waypoints(999)
Need to post-process for best results (internet connection)
GPS for GIS - Pros and Cons
• Trimble Pro XR
– Pros
Top-of-the-line GIS Mapping Tool
Real-time DGPS
Good under Canopy
Software is excellent
– Cons
Firmware will be changing?
Training involved
Right Tool for the Right Job
• Sorry… It’s a Multi - Tool Environment
– Garmin
– Trimble
• Need to learn enough about each system to know when to
use the tool
Mapping a trash can lid? - ProXR
Mapping a ATV trail - Garmin, Geo3 or ProXR
Mapping sheep from the air? - Garmin\PLGR
Mapping complex attributes? - Geo3 or ProXR
In Dense Canopy - PLGR / ProXR
Ideal GPS Toolkits
• Garmin for navigation, point collection
– Use DGPS system only if working along Gulf of Alaska,
Southeast, out on the water
• For Serious GIS
– Trimble Geo3 for open country mapping
– Trimble Pro XR for hostile environment (woods, city, lots of
What is Best?
• Ask Yourself
• 1) What are your accuracy needs?
– A) resolution of the other data types
– B) anticipated data for the future
• 2) Project Needs
– A) What are your GIS skills
– B) Is it just you dealing with the data
• 3) What part of the world are you working in?
• 4) How much money do you have?
Vertical Data (elevations)
The football of accuracy
Note Geoid 12a is current (2013) – don’t use Geoid 09 or 12!
Do not switch geoids in the middle of a job or comparing datasets.
In PFO, check the coodinates tab in the Export Utility and browse the
Geoid choices under Mean Sea Level/ Other Geoid choices to select the
right one (Alaska, Hawaii, CONUS). Always check Geoid settings
including units settings (options/units) in PFO and standardize units for
all distance, area and precisions measures. When you export data the
“inf” file will have the Geoid information. Use it for your metadata!
Take a Coordinate to Court
Basic GPS use for NPS Rangers:
Average Points
Use an antenna
Know thy datum and write it down
Use differential correction
Store the data electronically from the moment it is
collected – do NOT write it down.
Accuracy Aids
David Wilson’s GPS Accuracy Page:
The Alaska Way (hand-outs)
Update GPS devices to local time - Keeps you square with
filenames, photolinking, etc.
Batteries- check ‘em and charge ‘em!
SOP from pre-field prep through fieldwork, post processing
and export to make good shapefiles and metadata
Groundhog Mapping model ddf
GPS myths dispelled The Alaska Way
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