GPS Mapping Guide

1.0 INTRODUCTION

The purpose of these guidelines is to describe recommended procedures for the use of the Global Positioning System (GPS) in creating WPT(Waypoint), Mark and Track Files.

It is hoped that Riders will take opportunities to revisit sites to re-record and double check GPS coordinates during the course of their own ride.

2.0 SURVEYING WITH GPS

All GPS surveying techniques are based upon observations of radio signals from a network of orbiting satellites. These signals are processed to compute station positions by trilateration: the positions of the satellites and computed ranges are used to determine the antenna position. These positions are normally computed in an earth-centered Cartesian coordinate (x,y,z) system, which can be converted to geodetic curvilinear coordinates (latitude, longitude, and ellipsoidal height). With the addition of a geoid height model, orthometric heights can be computed.

The accuracy of a GPS survey is dependent upon many complex, interactive factors, including:

• Observation technique used

• GPS receiver age and quality

• Amount and quality of data acquired

• GPS signal strength and continuity

• Ionospheric and tropospheric conditions

• Site obstructions

• Satellite geometry, described by the Dilution of Precision (DOP)

Added to these are topographic errors in paper maps and ‘user’ error, often found in transferring data from one medium to another. The purpose of these guidelines is to eliminate as many sources of error as possible and create a reporting standard for submitting GPS coordinates with site records.

It is important that users have a basic understanding of how GPS works before collecting data. All GPS units require the user to adjust certain settings and how these are configured can seriously affect the quality of coordinates. Consult your manual for your type of GPS.

3.0 EQUIPMENT STANDARDS

It is assumed that most Riders will be using recreational GPS receivers, such as those manufactured by Garmin or Magellan. Recent tests have demonstrated that recreational GPS receivers are of a high enough quality that they are acceptable for recording coordinates in open areas where the zone 15° above the horizon is not obstructed. In areas of dense forest cover, or adjacent to high or abrupt topographic features such as cliffs, satellite signals may be blocked or interfered with. Higher quality receivers (such as those produced by Trimble) are usually associated with stronger signal strength and permit a greater range of user settings.

A good quality hand-held GPS receiver will have the following features:

• A multi parallel channel (preferably 12 parallel channels) receiver unit.

• The ability to store at least 500 waypoint positions in the receiver's memory.

• The ability to list waypoint positions and their distance and direction from the current position.

• The ability to upload/download waypoints and routes to a computer (most GPS dealers have their own or will recommend software that will help you do this).

• The ability to store routes, especially if you are recording your movements, roads, rivers in the field. Most will allow you to store 10-20 routes.

• Consider a position-averaging feature; this helps improve the accuracy of waypoints.

Note- my experience has been that the newer models of GPS have a considerable stronger signal reading strength and can get accurate readings in heavier cover than models 5 years ago(2002 or before).

Software and equipment to export your waypoints to a PC.

• A data cable. These sometimes come bundled with the GPS unit.

• Software. Most GPS manufacturers sell software that allows users to download and export data from their GPS units. Free downloads are available on the web as well.

4.0 GPS CONFIGURATION:

Most recreational GPS receivers permit users to configure some settings. Ensure your GPS is configured to the following settings:

5.0 Datum:

If your GPS does not permit datum configuration then it will be factory set to WGS84, which is the standard for the GPS satellite grid. When downloading coordinate data from your GPS please note that for most recreational GPS receivers the format is set at WGS 84.

5.1 Accuracy Estimate:

Most recreational GPS receivers today typically provide an accuracy estimate directly related to satellite geometry and expressed in feet/meters. Coordinate readings with an accuracy estimate of more than 10 meters/30 feet should be discarded.

Some older models of GPS receivers provide an accuracy reading called Position Dilution of Precision (PDOP). If your GPS unit allows PDOP settings ensure that it is configured according to manufacturer specifications and when recording coordinates an accuracy reading no greater than 6 is acceptable.

5.2 3-D Mode:

All GPS readings should be taken in the 3-D mode, requiring signals from a minimum of four satellites. If you are not getting a 3-D mode right at the site you want to mark/waypoint ride a little away in each direction till you have 3-D mode. An accurate 3-D mark/waypoint that is near the site but not right on it is more important than a 2-D mark/waypoint that is right on the site but could be totally wrong. When not in 3-D mode your waypoint/mark could be hundreds of feet or more wrong.

5.3 Signal Strength:

A minimum signal level of 6, or the manufacturer's recommended level for good signal quality. Some models, for example Garmin provide only a bar graph indication.

5.4 Satellite Elevation:

A satellite elevation angle of 15 degrees is typically regarded as a minimum. Note that dense forest cover or abrupt topographic features will affect signal strength and angle of visibility.

The actual process of creating a waypoint for a position will vary depending on the type of GPS receiver you are using. The following are general guidelines and tips for creating a useful waypoint with any GPS receiver.

• Be sure you have initialized your GPS receiver according to your individual unit's instructions. The first time a GPS unit is used in a new location (or whenever it has been moved 500 miles from where it was last used), the unit will need up to 15 minutes to orient itself. The more often you use the unit in its new location, the faster it will receive satellite data and record positions. Batteries should be checked frequently and replaced or recharged as needed.

• When you have located a site you wish to record make sure you have as clear a view of the sky as possible. Most newer multi-channel GPS receivers need to have a fairly open view of the sky. Leaves and branches of trees and low-growing shrubs cause interference and slow the reading process down but multi-channel receivers should still function in these conditions. If your site is significantly below the level of the ground the reading process will slow down due to the limited view of the horizon. Most GPS receivers will not receive satellite data indoors.

• Start your GPS according to the manufacturer’s instructions. Most GPS receivers operate in different modes. The receiver will usually tell you something about the mode in which it is operating as it locates satellites. Ensure settings conform to those in s. 4.0 above.

• Leaving your GPS unit turned on for several minutes (15 minutes or more are best) allows it to take multiple readings which are then averaged by internal software. This provides a more accurate location.

• Record your position as a waypoint. There are various ways to do this, and your unit may have a Position Averaging feature which will allow you to take a slightly more accurate waypoint reading, so follow the instruction of your individual unit.

• Give your waypoint a record number. Most GPS units allow you to give your waypoints some kind of text or numeric identifier so that each waypoint is easy to index in the GPS unit's memory. Keep track of your individual waypoint's ID and record it in your notes. A simple notepad will work. Don’t rely on your memory to remember what waypoint was what site. We use a handheld voice recorder to take notes and match the waypoint number to the voice recorder message number. This allows for faster note taking and more descriptive notes.

7.0 REQUIRED DATA FOR SUBMITTING SITE LOCATION COORDINATES (waypoints, marks, Tracks)

With your tracks and waypoints please submit as much of the following information for each site as you know.

-Description of area mapped. Example – Site name, State, park name, Trail name, any info riders should know. Like closers, rules, tips etc.

Be sure you have initialized your GPS receiver according to your individual unit's instructions. The first time a GPS unit is used in a new location (or whenever it has been moved 500 miles from where it was last used), the unit will need up to 15 minutes to orient itself. Batteries should be checked frequently and replaced or recharged as needed.

When you have located a site you wish to record make sure you have as clear a view of the sky as possible. Leaves and branches of trees and low-growing shrubs cause interference and slow the reading process down but multi-channel receivers should still function in these conditions.

Start your GPS according to the manufacturer’s instructions. Most GPS receivers operate in different modes. The receiver will usually tell you something about the mode in which it is operating as it locates satellites. Ensure settings conform to those above.

When your GPS receiver has four satellites in view(3D-mode) it is ready to calculate your position. If you just turned on your GPS leave your GPS unit in the center of the site for at least 15 minutes to allow it to average a more accurate position.

Record your position as a waypoint. There are various ways to do this, and your unit may have a Position Averaging feature which will allow you to take a slightly more accurate waypoint reading, so follow the instruction of your individual unit.

Give your waypoint a record number. Most GPS units allow you to give your waypoints some kind of text or numeric identifier so that each waypoint is easy to index in the GPS unit's memory. Keep track of your individual waypoint's ID and record it in your notes

-Description of area mapped. State, park name, any info riders should know. Like closers, rules, tips etc.