The Value in GPS Error

Three little letters, "EPE", have as much influence on your daily fishing action as another important set, "GPS". I am pretty sure that everyone, for the most part, knows that GPS stands for Global Positioning System and some of those folks know that there are a series of 31 satellites orbiting the earth at an altitude of 20,180 kms, steadily transmitting precise location and time information, which our GPS receivers use to calculate our position anywhere on earth. On the other hand, I'd hedge a small wager that more folks than not, don't realize that EPE stands for Estimated Positioning Error. There is lots of information out there concerning EPE but to break it down in a basic way, it is a calculation, based on multiple variables, that estimates the potential error in your GPS position being displayed on your screen. If you have an EPE of say 15 feet, it does not mean that you are 15 feet off of your exact location but rather, your position fix has a calculated probability of being within a 15 foot circle. Therefore, you could in theory then, have a 50% chance that your fix is within the 15 foot circle or a 50% chance that it is outside of that 15 foot circle. Please note, that there is no set formula for everyone to use, there are three main ones generally in use so do not use EPE as an indicator that one unit is more accurate then another. Even similar units from the same manufacturer could show different EPE values in the same spot. For posterity sakes, the three main formulas as suggested by US National Park Service: EPE is Estimated Positional Error (horizontal) EPE = HDOP * URA (1-sigma) would give a measure of the 68% confidence circle, i.e., 68% of your position fixes would fall within and 32% would fall outside. EPE = HDOP * URA (1-sigma) * 2 gives a measure of the 95% confidence circle, i.e., 95% of your position fixes would fall within and 5% would fall outside. EPE = HDOP * URA (1-sigma) * 0.73 gives a measure of the 50% confidence circle, i.e., 50% of your position fixes would fall within and 50% would fall outside. This is also known in the literature as CEP (Circular Error Probable). * HDOP - Horizontal Dilution of Precision * URA - User Range Accuracy is a quantity that is transmitted in the navigation message that is the predicted (not measured) statistical ranging accuracy. So why all of the technical jargon and why did I want to learn about and try to utilize the EPE data to my advantage? To more quickly and hopefully, more accurately find my favourite fishing spots on a spot for example. Very expensive commercial and survey level GPS receivers, with software enhanced data back at the office, can get a point on earth down to centimetres or less. However, our less expensive, recreational receivers will by design have an accuracy of a few feet, at best. Now throw in the EPE value and the probability that your accuracy has a definite possible chance of being in or out of that EPE identified circle, you can see that we are never exactly where we think we are. That said, it is worth a little extra effort to try and minimize your inaccuracy so that when we factor in product level errors and EPE errors, we're close enough to where we want to be to maximize our fishing effort via our electronic reference. So you make a spot on the map with an "X" or waypoint. Think of that spot as being at the centre of a circle. Right from the moment you push that waypoint point, your circle could be 3 feet or even 10 feet etc. Now, your EPE is telling you that statistically, you could have certain chance of being within another circle of that defined size, let's say 25 feet. So your 3 foot circle could be anywhere inside that 25 foot one or it could also be outside that 25 foot one. I know, you're thinking Seinfeld right now, yada yada yada :) but there are a few things you can do to help make your accuracy improve. Although all modern GPS and combo units use accurate, internal antenna's, you can increase your odds by using external antennas. They can be positioned better thereby creating a more usable reference point. Also, you can obtain even more accurate or faster external antennas by comparison. As anglers, we spend the majority of our time standing on the front deck and fishing from there. It only stands to reason then that we can increase our waypoint accuracy by having an antenna up there with us. If you are in a 21 foot boat and marking rocks up front on your sonar but your antenna is at the console (10 feet away) or transom area (20 feet away), you are introducing a definite circle of area before the other two mentioned circles of error come into play (EPE and recreational grade receivers). Based on that fact, it is best to install an external antenna up on your bow and as close to your bow tip as possible. By doing this, your antenna data is above or within a foot or two of your transducer, allowing you to mark underwater marks more accurately. Also, when moving your boat around to find a waypoint, the tip of your bow is getting there first so it increases you accuracy overall by making that your reference point. For running the lake and seeking out waypoints, it is best to have your bow and console units both reference that bow mounted antenna. Again, when motoring around, the bow is your reference point. In today's world, personal mapping is all the rage and so it should be, it's one of the most exciting things out there right now. Humminbird's Auto Chart Live and Auto Chart Pro are the leaders in this segment and here too we can take steps to improve that map making accuracy. By mounting an external antenna back in the splashwell area, we can get our antenna reference to within a foot or two of our recording spot....our transducer. So all of the data points used are more closely tied together, we can now say with more confidence that at point XYZ, the depth was said value. Now when our data is complied, our displayed chart is more accurate from a standpoint of relative position. When mapping, have your main mapping unit reference that stern mounted antenna to achieve this added layer of accuracy. One last thing we can do is to install a heading sensor into our system. This electronic compass helps to better identify, on our unit's screen, the direction of our boat at all times. Gone are the days of drifting backwards and your map turns around showing you travelling forward in the wrong direction. Now your boat will still point in the same direction you are on the map while the display now shows you drifting backwards. This helps you to better realize and focus in on a waypoint regardless of how your boat is being moved towards that waypoint (big motor, trolling motor, drifting). Based on this last point, it is highly recommended to have one of your external antennas contain a heading sensor such as the Humminbird AS GPS HS device. Have both your bow and console units obtain the heading data from that antenna at all times. Before former president Bill Clinton signed off on the order to turn off SA, the US government used that offset to purposely make our units even more inaccurate. Relative to the days of S/A (Selective Availability), when our error in position could be up to a couple hundred feet, today's units are very accurate but as humans and as anglers looking to focus in on certain smaller features, anything we can do to help increase our accuracy / repeatability can only help us achieve better results out there on the water. Good fishing!