GPS Investigations

GPS Investigations Begin by Knowing Your Needs

Device operations should be consistent with mission intent

GPS Investigations

Your selection of a GPS receiver for field work involves many aspects that must be considered before you purchase a receiver and put it into use. GPS investigations need to consider what you will use the data for and the environment it will be used in. We offer our experience to visitors of this site in some things to consider before you invest in the GPS Receiver and then learn how to implement the data into the workflow of your operation.

Initially developed by the U.S. military, the satellite constellation provided earth positioning based on satellite interactions. Precise locations were allowed only for military users. Non-military could post-collection deferentially correct their positions, but it generally took a day to calculate solutions. On May 2, 2000, “Selective Availability” was discontinued to allow consumers use of accurate positioning in real time. This marked the consumer-grade devices which flooded markets.

In today’s market-place, GPS users consider three common application types:

  1. Navigation only (Automobile and/or Aviation),
  2. Points, Lines, and Polygons from the field to the computer,
  3. Points, Lines, and Polygons from the computer to the field.

Navigation GPS

Navigation Global Positioning System devices have been integrated into automobiles to aid drivers when traversing routes. Commercial aviation has used these devices since selective availability was removed. However, these navigation devices are not specifically designed to precisely pinpoint location of the vehicle. Of course, commercial aviation uses specific location specificity terms for landings and takeoffs. The automobile GPS, built into the vehicle’s dashboard, is concerned with items of interest ‘near’ the vehicle’s location.

The automobile manufacturers considering “self-driving cars” has changed the ‘precision needs’ of navigation GPS. Obviously, precision takes on heightened importance for that application. In this review of GPS units, navigation GPS units will not specifically be considered. However, users will discover that hand-held units can ALSO be used for navigation purposes. Used at least for the traditional “Driver Controlled” automobiles. Read on…

Points, Lines, and Polygons from the field to the computer

Data From Your GPS Investigations

GPS Investigations involving collection of points, lines, and polygons in the field to be used in GIS analysis is common. Readers will recognize that while field data location is important, its precise position may not be critical. This may be considered when thinking about the location of a salmon Redd. While its location may be very important for management purposes and to show habitat use, the precise location is not. If the GPS unit records the location within 3 meters of its actual coordinates, the accuracy is adequate.

On the other hand, the precise location of a bridge’s support pillar may need to be recorded to within 1 millimeter. The GPS investigation needs to consider data use versus data cost. The consumer grade GPS device may cost around $200. The engineering grade GPS may cost $25,000. Of course, there are ample levels of accuracy and cost between these bounds. But consider, the upper end costs extend above this range.

Data Costs Time and Money

Users of these devices should consider the cost of the data they collect. This is in reference not only to the cost of the GPS unit, it includes operations in the field. Most consumer grade GPS are hand-held, but they can be mounted on a staff. On the staff, some Consumer Grade GPS units will accept an external antennae. The external signal collector can decrease signal degradation substantially. Multipath signal disruption happens when the body of the field crew blocks the satellite signal bound for the GPS unit. Keeping the GPS unit under a coat, inside a vehicle, or in a vest produces the same degradation. Trees, cliffs, and even heavy snowfall will degrade the signal if it is between satellite and unit. An external antennae can solve ‘many’ of these problems.

Elevated on the staff, the GPS antennae collects data from above the operators head. The antennae with a clear view of the sky will perform best. Besides, a staff impaled into the ground will hold the antennae still. That makes the point collected by the GPS receiver much more accurate.

Garmin 60CSx Staff external antennae

Data Needs vs. Wants

GPS investigations need to consider the data you will collect. Are you seeking points across the landscape? These are often collected in forest management applications when crews record the location of property corners or sample-plot centers. The GPS Points are needed for GIS data and maps. You want to know about how these points are associated with other land features. This is how to get those data points. Ask yourself, how will the seospatial data collected be used?

Many times, straight lines connecting points are made in GIS programs. So, detailed GPS line collection is generally not needed. And, it costs more to attempt them this way. Generally, the quality of GPS lines is inferior. Other times, the intended lines are not straight point-to-point connections. This is when the GPS can be used to make the lines through a series of irregularly placed points. Make the GPS points high quality points. Use the GIS line feature to make sure you connect them in the right order.

Another consideration of field use GPS data is to consider using the data collection points as a navigation destination. Points in the GPS unit can be navigated to: make the point a destination, then navigate to it. The user may be walking, following a compass bearing, and watching the distance. When the point is reached, the GPS confirms arrival. It is GPS navigation without the vehicle! Best of all: it works.

Although a GPS Unit looks like it has no moving parts, it does experience wear and tear. Units are dropped, they impact other objects, and get rained on. These events generally do not cause immediate damage. But in time, they add up, and point accuracy is compromised. This is why we recommend finding the “Right GPS Unit for the Right Job”. If the Garmin GPS (Recreation Grade) is sufficient to document points or to use for navigation, then use it. If precise attribute position is required, then use that Trimble GPS (Survey Grade).


A strategy we have long used is explained with ‘familiarity’. We want our people to be really familiar with the GPS units they use for work. For us, it meant allowing people to take the units home, on a fishing or hiking trip. Use the unit and get familiar with it.

Travelling with the family? Take the GPS unit to make sure your route is planned well. If you need to use the data storage to find a restaurant, hotel, or park, then do it! Become familiar with the unit. Want to take photos using the Trimble unit’s camera? Do it. Do you see a trend here? Of course you do…

Make the mobile phone active on the Trimble Juno unit so data can be activated while in the field. Then it can be forwarded with a data-connection through e-mail or as a drop-box FTP connection. Allow that photo and phone combination for the user’s family trip. They will be familiar with the operation when needed for a work related transfer. That is priceless.

When you use that same unit for recreation and at work, you will be comfortable with it and get the most out of your time. You also get the highest value from the technology you adopted.

Is there a down-side to this approach? Sure there is. One employee had a pet parrot who traveled with him and one day the bird ate the key-pads off the Garmin GPS which was mounted on his dash-board. Another staff member accidentally left the GPS unit on the hood of his truck and ran over it as he drove out of the fishing boat launch area.

He Devil Wilderness

Sure, stuff like that happens, but the same things could happen while working on a project. I was working on a project for a client to complete a ocean shorelines management plan. The effort involved making some detailed positional recordings of basalt rock outcrops along the shoreline. I positioned my staff mounted Trimble Juno (3 Series) along the shoreline to record an outcrop position. I watched the screen to photograph the basalt formation and a sneaker-wave uncoiled right where I was standing. It washed my GPS unit, staff and all out to sea. I was not prepared to chase it down. Thankfully, I routinely sent my data through the mobile connection to my FTP drop-box so little data were lost. I was fortunate to have a Garmin Map 76 unit in my vest and I was equally proficient using it. Although the data could not be post-collection defferentially-corrected, it made the rest of the remote GPS investigations possible.