Configuring GPS Unit & Mission Planning

Discussion

Once the necessary office planning hurdles have been overcome, the GPS unit(s) need to be configured, uploaded with necessary files, and the whole system tested. Also introduced here are methods to determine the best days and times to venture out into the field.

GPS units vary widely, but all have some configuration settings that can be adjusted to help with field data collection for different environments and different projects. In field projects with multiple GPS units, configuration standards should be developed and applied to all of the GPS units. Documenting and adhering to the configuration standards will help when the project and finalized GIS datasets are documented. READ MORE >>

It is very important for users to become comfortable with manipulating GPS unit settings and understanding their different functions. The best way to do this is to read the GPS unit's manual (egads!!) and talk to users who have operated a GPS unit in the same geographic area as your project. Some common setting to be aware of in any unit are:

Now is also the time to upload files to the GPS unit. READ MORE >>

Depending on the GPS unit you may be able to upload configuration files, background georeferenced images, background georeferenced vector data, data dictionary files (Trimble), attribute forms, and/or waypoints. Uploading usually requires one or more compatible software packages installed on the designated laptop or PC and the cables to the serial (COM1 or COM2) port or USB port. The same transfer setup will be needed after GPS collection to download the data.

GPS mission planning is a critical component of any project involving GPS under challenging conditions (under canopy or in mountainous terrain) and can be performed independent of the GPS unit type. READ MORE >>

Mission planning allows you to identify the best and worst time periods for GPS data collection at any location by performing an analysis to determine visibility and configuration quality of GPS and geostationary satellites. Utilizing this tool will help you plan more productive field days and work times. Some software packages accompanying GPS receiver units have this utility built-in or you can download Trimble's Planning software for free (see Links section).

Test everything before starting your data collection efforts in earnest. READ MORE >>

This includes all aspects of GPS data collection. Assemble the GPS units, run a mission plan for the day of the test, and use the configuration settings as determined for your project. Collect features while utilizing the attribute collection method established. Return to the office to download and post process the trial features, see the GPS Data Post - Processing step for more details. A complete trial test will insure the GPS unit plus cables and associated equipment are all present and functioning correctly. It will also help to get GPS data collectors familiar with the process used for data collecting and post-processing.

Watch outs!

Working with data in NAD27 Datum - Your park's GIS data may be displayed in a projection based on the NAD27 Datum.  Anytime you transfer data to/from a GPS receiver the software you are using must be told the correct projection and datum of the transferred data. Once the data is stored in the GPS receiver, the user can alter the style of coordinate format (UTM, Lat/Long DMS etc) to any preference. Some GPS receivers may not be able to display accurately the same coordinates in NAD27 due to the complexity and file size of the datum transformation. These errors may be on the order of 1 - 10 meters depending on your location.

Mission Planning - Planning your GPS day with a fresh almanac and planning software will keep surprises to a minimum and allow for more efficient and economical field surveys.  Skipping the planning step often comes when you get into a hurry, need to collect today and feel there are plenty of satellites. Although we are seeing excellent world-wide coverage, there may be satellites that are not transmitting over your area of work and may degrade your on-ground precision and accuracy. If your GPS system is capable of transferring an almanac to the PC, take the time to download the almanac at every download opportunity. Once a fresh almanac is downloaded to the PC, use Trimble's free Trimble Planning software (www.trimble.com/support.html) to plan and schedule GPS field sessions.

Glossary of Terms

Coordinate System - A coordinate system is a number used to locate a point with a three-dimensional reference frame. Map coordinates are often expressed in UTM or latitude and longitude (sometimes called geographic). Within the geographic coordinate system, there may be different ways to express the same coordinate, such as Degrees Minutes Seconds (DMS), Decimal Degrees (DD) and Degrees Decimal Minutes (DM).

Datum - A mathematical model representing the earth's sphere used for Geodetic calculations. Some models are create to represent specific continents. Example: North American Datum of 1927 (NAD27) was created and calculated in 1927 to help represent the earth's shape and is best suited for calculations in North America.

Elevation Mask - On some units you can configure the elevation mask on your data collector to ignore satellites that are low in the sky. When a satellite is low on the horizon, the satellite signals must travel a greater distance through the atmosphere, resulting in a lower signal strength and delayed reception by the GPS receiver. Low-elevation satellites tend to yield noisy data. Position data should be collected using only satellites that are at least 15 degrees above the horizon. (Source: GPS Pathfinder Office 2.90, copyright 1996-2001, Trimble Navigation Limited.)

EPE - Estimated Positional Error for the horizontal direction. A more complete explanation can be found at www.nps.gov/gis/gps/WhatisEPE.html.

PDOP - Position Dilution of Precision (PDOP) is a measure of the current satellite geometry. The lower the PDOP value, the more accurate the GPS positions. (Source:  GPS Pathfinder Office 2.90, copyright 1996-2001, Trimble Navigation Limited.)

Projection - A method of representing the earth's three-dimensional surface as a flat two-dimensional surface. This normally involves a mathematical model that transforms the locations of features on the earth's surface to locations on a two-dimensional surface. (Source: Association for Geographic Information - GIS Dictionary)

SNR - Signal-to-Noise Ratio (SNR) is a measure of the strength of the satellite signal relative to the background noise. Accuracy degrades as the signal strength decreases. (Source: GPS Pathfinder Office 2.90, copyright 1996-2001, Trimble Navigation Limited.)

Links (Step-by-Step How To)

Configuration Setting Guidelines and Examples: Recreation\Consumer Grade GPS Specific Information: Mission Planning:

Site Suggestions. Contact ifer_mccollom@nps.gov if you have any suggestions or possible Step-by-Step How To links.