As we know, absolute accuracy of standard GPS (not DGPS etc.) is approximately 15m. Interestingly, this is not the relevant measure when talking about the accuracy of timed laps using GPS triggers. Relevant for this discussion is the "drift" we get from our GPS device when standing still, i. e. the relative accuracy. To make this clear, think about an absolute misplacement of e. g. 10m of a specific GPS device exactly in the direction of driving. With a relative misplacement of hypothetically 0m, LapTimer would trigger lapping 10m late - but would also stop 10m late as well. This gives some error to intermediate times but none to the overall lap time! It's a different story with relative accuracy.  If we get the start 10m late and the stop 10m early, we have shortened the lap by 20m which, at a speed of e. g. 75 mph, corresponds to 0.6 seconds. Happily relative accuracy is much better than this. We did not find exact numbers in the literature, but looking at our consumer GPS device, we see a drift smaller than 0.001' standing still (this is the precision you see in LapTimer, internal calculation is even smaller: 0.0001').

This means, that - in the worst case - you get your lap wrong by approximately 0.06 seconds (assuming 75 mph).

Another variable is on the update frequency of GPS devices: Standard consumer devices (especially all Bluetooth based devices) come with an update frequency of 1Hz, i.e. once a second. The trick is that when LapTimer recognizes that you passed a trigger it calculates the time taken from the trigger to the last reported position (using current speed) and adjusts the time taken accordingly.  Our Lap Timer comes with a 5Hz receiver i.e. five times per second that provides even greater accuracy.

As you see, accuracy is a complex thing, but steps are taken to get it at least to 0.1 seconds.

The most powerful feature of LapTimer is its integration with GPS input from NMEA devices. The idea is, to replace user operated events such as pressing a button to start or stop lapping, by automated events. For example, if LapTimer knows about the coordinates of the finishing line of your track and receives continuous input from a GPS device, it is able to track when you cross this line! LapTimer implements this as "GPS Triggers". In the event of passing a trigger's location, you can define an automated start / stop or intermediate timing operation for LapTimer to use.

How do I define the Start and Finish GPS triggers?

• It is easy to define triggers with laps already recorded.  We advise using a warm up session or first session to manually record a full lap to gather all the GPS coordinates for the track you are at.  To do this, always power on the GPS receiver before starting up Lap Timer.  Next start up Lap Timer and select "Bluetooth" for the GPS mode.  Have Lap Timer on either Classic, or Biker View.  Head out on to the track and at any point on the track that is comfortable simply tap the timing screen with your finger (not near any other buttons on the screen, but right on the timing view) to start timing for that lap.  Ride all the way around the track on your race line.  Be sure to pass the point where you started the timer and then tap the screen again to stop the timer for that lap.  This will create one complete lap in the database that can be used to set all of your triggers.
• Once you are off the track, select one of your manually recorded laps and switch to the "GPS Data Recording". Move the cursor to the point you would like to use for the Start/Finish and hit the "+" symbol. The "Add Trigger Dialog" will appear. Either set this trigger as a "Stop / Go" (round trip tracks) or a "Go" trigger type (tracks with starting line different to finishing line).  In case you choose "Go", move the cursor to the end of the track and hit "+" again. Define the corresponding trigger type as "Stop". Having done this, LapTimer will automatically start / stop the next time you pass the trigger positions.

In order to get sector times from Lap Timer you will need to add some Intermediate triggers.  In order to add Intermediate triggers you will need a previously recorded lap for the track that you want sector times for.

• Choose a recorded lap from your lap list
• Enter the "GPS Data Recording" screen
• Move the cursor to the point you would like to use for acquiring a sector time and hit the "+" symbol. The "Add Trigger Dialog" will appear.  Set this trigger as a "Intermediate" trigger.
• Where you set your Intermediate triggers is where timing will stop for that sector.  Timing for that sector will start at the closest previous trigger, either Intermediate or Stop/Go.

A picture is worth a thousand words so here are some of the views shown with explanations of each component.

The Racer's view includes the most comprehensive information for regular lapping. As a special feature it includes a kind of "Ghost Mode" known from Console Games. Using this Ghost Mode, you are able to compare your current time to a reference lap (last, best, or any selected) at any time in track. Furthermore it features information on the last cornering an straight speed, important indicators for fast laps.  In case you run regularity in terms of repeated laps, select your first lap as the "GPS Reference Lap" (select lap in lap list and enter the Lap Details dialog). After having selected the reference lap, your current lap will be compared to this reference.

The late beep has several reasons. LapTimer detects a trigger after passing it.  The real time is calculated by interpolatig the last two fixes (one before the trigger and one after).  Also, the sound output takes some time, so LapTimer beeps after all other actions have been taken (interpolation calculation, writing the database, checkpointing the database etc.).