Angle measurement. Your eyes are surprisingly good at lining up the smartphone with the line with good accuracy. However, in a pitching boat, or with poor light, line angles are sometimes hard to gather. It is important to hold the smartphone parallel to the boat’s direction of travel and to the line. If you have trouble, take several measurements at each line length and then inactivate or delete inconsistent measurements. The optional Autocline device can make angle measurements easier and possibly more accurate.
Speed control. You should try to keep your boat at the same constant speed while calibrating and while trolling.
Direction control. If the boat changes direction while calibrating or trolling, the angles of the trolled line may change.
Current. A river current or tidal current can cause you to misunderstand the actual speed of the line and lure through the water, as a GPS shows speed-over-ground, not speed through the water. Trolling Angles can help measure the current’s speed and direction, and can correct for it while trolling.
Curve Approximation. A variety of different algorithms and curve fitting techniques were evaluated during the development of the app. A technique that mimics real world physics with an approximation to a smooth curve was chosen. With reasonably accurate measurements spaced at 10 feet each, almost any reasonable smooth mathematical function would lead to less error than the fish would notice.
Mistakes. The most common source of error is just user error, like manually entering the wrong line length or angle.
External Factors. There are some factors unrelated to the app to keep in mind while trolling. Your depth-finder transducer is probably mounted a foot or more below the water surface, causing it to misstate the depth of suspended fish or the bottom. Objects at the outside of the transducer’s cone appear to be deeper than they actually are. Depth charts are not always precise, and water level fluctuations can cause them to be off at times.
