GPS Unit & Project Needs Assessment

Discussion

This discussion is just a brief introduction to the considerations and choices you need to take into account when choosing an appropriate GPS unit(s) for your field project work. The topic of "What is GPS" and different GPS units specifications are not covered, the Links section has many documents and MS PowerPoint presentations with that information.

At the beginning of your project ask yourself the following 4 questions:

1) What features (real world items) will I be mapping? READ MORE >>

You'll need to consider the features you are going to collect for your GIS database to help guide you in your choice of GPS unit. Generally speaking, features are divided into three categories; point, line, or area features. For example, a telephone pole is a point feature, a road is a line feature, and wetland is an area feature. Some GPS units will record in the field all three types of features while other units require converting to features after the field. For instance, point features collected in a recreational\consumer grade Garmin unit are recorded as waypoints and line and polygon features must be created from tracklogs or routes. Look at a GPS units specifications to determine what are its feature capturing capabilities and determine if features can be directly recorded in the field or not.

2) What are my project accuracy needs? READ MORE >>

Accuracy can vary widely depending on the GPS unit you choose and the methods/configuration of your GPS. Accuracy needs should be based on the size of the smallest feature to be mapped and the resolution or scale of other datasets used in conjunction with your GPS collected data. Sooo... What is the size of the smallest physical thing (feature) you want to map? What is the scale of most of the datasets in the GIS data structure you inventoried?  National Map Accuracy Standards dictate the accuracy (in feet or meters) on the ground for common scales that a GPS unit must meet (see the Links section for this file).

After determining the project accuracy need, look at which GPS units achieve that measurement consistently and how. In some cases a GPS unit can only achieve the accuracy consistently if the data is differentially corrected (referred to as DGPS) to take out atmospheric interference. DGPS can be achieved during data collection (aka real time) and/or after the data in downloaded to your PC (aka post-proccessing). Look at a GPS unit's specifications to determine if it has the DGPS capability with either or both methods. In some cases, you will need to purchase extra equipment or services to collect DGPS data.

3) What GIS resources (skills) do I have available? READ MORE >>

Your comfort level with GIS technology or your group's GIS skills plus the time to edit the data after downloading should be a consideration when buying a GPS unit. Some knowledge of GPS PC based software or GIS software will be necessary in correcting errors in your field data. If you have a recreational\consumer grade GPS unit (i.e. Garmin) with waypoint and tracklog collection capabilities, you may need to spend time creating features and inputting attributing information from hardcopy field notes after returning to the office. Look at a GPS units specifications to determine if the out-of-the-box capabilities include capturing the attributes electronically and outputting to popular GIS features and formats.

In some cases, you can purchase additional software or equipment to make data collection and export easier. For instance, almost any GPS receiver can be connected to a CE based handheld device with data collection software that allows for point, line, polygon, and attribute data collection. See the Exporting GPS to GIS Format step for more details on export information.

4) How much money do I have? READ MORE >>

Sometimes it all comes down to the money... The cost and capability of GPS units can be lumped into three Grades. In the National Park Service, the recreational\consumer grade has been utilized for Incident Command Structure (ICS) incidents and facilities management MAXIMO data capture, mapping grade for natural resource and cultural resource data capture, and survey grade for boundary issues with the LANDS program.
This is not an exhaustive list & does not include additional hardware or software available. Recreational\Consumer Grade Mapping Grade Survey Grade
Cost ~<$1000 ~($2000 - $10000) ~>$10000
Features waypoints and tracklogs/routes point, line, and area generally collected as positions and processed later
Horizontal Accuracy (in ideal conditions w/ DGPS) ~1 meter and up submeter to ~5 meters millimeter to decimeter  (Vertical accuracy - centimeters)
Attribute recording electronically only with additional hardware & software in most cases yes
Amount of work to merge with GIS Datasets in the depends on if you use additional hardware & software while collecting moderate varies
Receiver examples Garmin handhelds, Trimble Pocket Trimble's ProXRs, GeoExplorer series, & GeoXT Trimble 4600, 5700

Watch outs!

Recording attributes in the field. Be aware that some units do not provide out-of-the-box capability for electronic attribute capture while data collecting. In these cases, all users will need to know standard attributes to record on paper per feature before data collection begins. Another option is to purchase additional hardware and software to make electronic attribute capture possible (example: Attaching a Garmin GPS receiver to a PDA with ESRI ArcPad software will allow for attribute capturing through screen forms).

GPS units accuracies can vary widely. Are you mostly mapping under canopy or in thick vegetation? Some units are more susceptible to signal degradation under forest cover and therefore the data will be less accurate or accuracy may vary from feature to feature. A GPS unit's ability to perform in adverse topographic and vegetative conditions and its specifications should be looked at closely to make sure you choose the right unit for your terrain and vegetation cover.

GPS unit already available? Maybe you are at a park with a GPS unit already purchased and available. If this is the case, you will need to fully understand its capability by reading the specifications and finding out how it answers the four questions in the discussion section to be able to adapt the GPS unit available to your needs.

Glossary of Terms

DGPS - Differential GPS is a data collection technique that uses an extra GPS receiver and some complex calculations to increase the accuracy of GPS positions. It is based on the fact that any pseudorange errors in a GPS signal are common to all receivers within a radius of several hundred miles. Differential GPS can provide accuracies from submeter to around five meters. (source:  GPS Pathfinder Office 2.90, copyright 1996-2001, Trimble Navigation Limited.)

post-process - One type of DGPS. Differential corrections are applied in the office after downloading the collected GPS data files and base station (an extra stationary GPS receiver) files to an office computer.

real time - One type of DGPS. Differential GPS corrections are applied at the time the data is being collected in the field. Additional hardware is usually required to receive the real time signal from a base station. (source:  GPS Pathfinder Office 2.90, copyright 1996-2001, Trimble Navigation Limited.)

resolution - The fineness of detail of an image or feature data. Usually expressed in units of meters or feet.

Links (Step-by-Step How To)

What is GPS?: Choosing a GPS unit: GPS Support Software for your PC, PDA and/or GPS Unit: More on Accuracy and GPS units: Learning about DGPS (Differential GPS):

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