Guidelines for Recording Handheld GPS Waypoints

Contents
Standards and Practices
Goals of this project
Getting Started: GPS basics
Recording geographic coordinates
Where to submit

Standards and Practices:

These guidelines have taken into account the Archaeology Data Service's GIS Guide to Good Practice http://ads.ahds.ac.uk/project/goodguides/g2gp.html, which promotes standard methodologies for collecting archaeological data, and the Dublin Core metadata standards.

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GPS allows us to determine a basic position of a feature or site. We can store these positions as waypoints within the GPS receiver and then use waypoints to plot these positions on digital maps or in GIS databases. Waypoint files are also useful because a GPS receiver can calculate distance, and direction to and between various waypoints/locations. We can also store routes (a series of waypoints) which can be used for navigation purposes, and can even be used to plot a trail, path, or road.

To date there are few repositories of geographic coordinates available to a wide audience that deal specifically with the ancient world, although there are an increasing number of individual classroom and research projects that make use of such geographic data. Since an increasing number of students and teachers, are traveling to the Mediterranean region with GPS receivers for their own study and research purposes, the Perseus Project and the Stoa Consortium have developed these guidelines to promote a standard methodology for recording and submitting data from these various projects. These standards would allow students, scholars, researchers or anyone working in the field to contribute to a growing permanent repository of geographic information that can be accessed and used by the widest possible audience.

The Repository of Ancient Geographic Entities (RAGE) will serve as a central organizational tool for geographic data recorded by various online projects and databases. Five activities will be available to RAGE users. Users will be able to: look up a project, look up a place name (to find matches or linked entities from any project), look up a single toponym (to find a single toponym which may include multiple names for that toponym in different projects), propose links between two names (submit that x and y refer to the same entity), and propose a link between a name and an entitiy (submit that x is a name for a particular entity already in RAGE).

For example, data stored in the repository could be accessed by and linked to a wide variety of schools, research projects, and digital libraries. Students, historians, or archaeologists, could download repository data to create their own maps and study projects, or load data into their GPS units to take into the field.

The following guidelines outline basic equipment needs and the recording and submission of GPS data. For more information about submitting geographic data to the Perseus Digital Library, or about registering your project with RAGE at the Stoa please contact Ross Scaife (scaife@pop.uky.edu), Neel Smith (nsmith@holycross.edu) or Robert Chavez (rchavez@perseus.tufts.edu).

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Some basic GPS equipment issues

What's the difference between a hand-held GPS and a GPS survey system?
A hand-held GPS receiver is low-cost ($100 - $500), available to the general public, and generally accurate to 100 meters. A GPS survey system is more accurate than the typical hand-held navigation unit. These systems can have an accuracy better than 1 centimeter and cost anywhere from $10,000 - $50,000. For some survey work Differential GPS (DGPS) systems can be used.

Why are most GPS units only accurate to 100 meters? Will my data be useless?
The Department of Defense (DoD), the U.S. government agency which established the GPS system, puts inaccuracies into the satellites' clock data which in turn adds inaccuracy into position calculations. This error is called Selective Availability (SA). The horizontal error with SA is specified to be within about +/- 100 meters (325 feet). Many users find that their readings are a bit more accurate than the 100 meter threshold, and some GPS units come with some limited error correction ability. There are some tricks GPS users can use to limit SA error, but only Differential GPS can reduce this error significantly.

The SA error is acceptable for the majority of GPS applications, including navigation and plotting site and feature locations on maps. We are not working at a scale which would require highly accurate plots.

SA is scheduled to be discontinued within the next 4 - 10 years, which should improve GPS user's acuracy to about 25 meters or better

Differential GPS (DGPS) DGPS corrects SA inaccuracies in the GPS system and allows us to position locations, features, etc. on a very precise scale. DGPS allows the GPS receiver to receive corrections from a reference station at a known position. Most new GPS receivers are DGPS ready, that is, they can be hooked up to a DGPS unit.

Real-time Differential GPS corrections are transmitted over radio link from radio beacons (reference stations). These can be difficult to come by overseas, but there are a number of private DGPS services that transmit real-time corrections in the Mediterreanean region.

Things you will need in the field:

1. A good hand-held GPS receiver. Look for the following features:
   • A multi parallel channel (preferably 12 parallel channels) receiver unit
   • The ability to store at least 100 waypoint positions in the receiver's memory
   • The ability to list waypoint positions and their distance and direction from the current position
   • The unit should be able to handle the UTM (Universal Transverse Mercator grid) data, esp. if you are using gridded topographical maps.
   • 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.
   • If you plan on purchasing a DGPS receiver at some point, be sure your GPS unit is DGPS ready
2. Software and equipment to export your waypoints.
   • 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.

A nice freeware package for downloading GPS data for Garmin receivers is Waypoint+ (Windows 95/98/NT)

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How do I take useful waypoints in the field?

1. 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.

2. When you have located a feature 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 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.

3. Fire up your GPS according to the unit'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. When three satellites have locked into your GPS unit, you will get a reading. Some units call this 2D mode because there is no elevation reading. If you have an older GPS unit you may only be able to take 2D readings.

   Multi channel units allow the GPS receiver to read an additional satellite. When a fourth satellite comes into view, the unit will be able to add an elevation reading thus increasing the accuracy of of its calculation. This is sometimes called 3D mode.

4. When your GPS receiver has four satellites in view. Check your DOP/PDOP Some GPS receivers have a display labeled DOP or PDOP. This reading will give you an idea of how accurate your readings are. The lower the DOP/PDOP the more accurate your reading should be.

5. 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.

6. 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 the Data ID field of your Site/Feature record.

What additional data should I record about my waypoints?

The data repository is essentially a database made up of the records that you submit. Remember, people who see or use your data may know nothing about the site or object you have recorded. The record you submit with your data, based on the internationally recognized Dublin Core model, helps describe your data in a standard way to potential users.

By including a Project and data record, you are ensuring that your data will plug in easily with the Digital Library database and conform to international standards of data collection and description.

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I have waypoints, how do I submit them?

Questions and suggestions can be sent to: Rob Chavez
Stoa/Perseus Project

Please send your comments concerning The Stoa: A Consortium for Electronic Publication in the Humanities to Ross Scaife (scaife@stoa.org). This document was published on: Tuesday, September 02, 1999