GPS Twin Odometers

Historic, Classic, or Vintage road rallies may permit digital dual odometers, clocks, and stopwatches. This app combines all three using timers and GPS devices built into Apple mobile devices.

GPS inputs are used strictly for distance measures along the rally route. Also, the displayed accuracy of two decimal places conforms to typical historic, classic, and vintage road rally restrictions.

However, it is possible to interpolate distance to three decimal places. There are tick marks to indicate the second decimal place number centered on the display as well as increments of +0.0025, +0.0050, and +0.0075. When touched, the Control button will pause the two odometers for 4 seconds to allow you time to estimate distance to thousandths.

Similarly, time measurements are limited to whole seconds or cents. Time, distance, and mode (discussed below) may be chosen at the start of a new rally by touching the Set button to access app Settings.

A set of buttons allows you to chose the active odometer. The leftmost button activates the lower "interval distance" odometer; the rightmost button activates the upper "total distance" odometer; the center button activates both odometers.

From time to time you may measure time and distance for a stage or a checkpoint and record data by touching the Control button. The time and distance will be stored in a log along with geographic coordinates.

Distance options include subtraction of distance when you travel off course. Direction buttons switch between forward (+), reverse (–), and park (0). Also, you may chose between miles or kilometers.

There are three modes of operation selectable in the app Settings: Regularity mode, Jogularity mode, and Lap mode. In Regularity mode, the stopwatch will be reset to zero each time the Control button is pressed. In Jogularity mode the time split will be recorded but the stopwatch will continue recording. In Lap mode, the tabular display will reset the stopwatch to zero when the control button relabeled Lap Reset is touched and each split time will be identified as a Lap in the table. Also, in Lap mode, the options to zero either of the odometers are removed, however the lower odometer can be set to zero at the start of each lap by touching the Lap Reset button.

Note: Use of the GPS Twin Odometers during any particular rally will be subject to the rules and instructions governing the rally and may define your category or class among participants. Please consult the Rally Master for approval of the use of this app.

For better GPS reception and improved accuracy, third-party wireless GPS receivers are recommended. This app has been tested with Bluetooth connectivity to a Dual XGPS150A GPS Receiver. This app works with both iPhones and iPads, even those without Cellular connectivity. However, there can only be one Apple device with Bluetooth activated within range of the Dual Receiver. Place devices not running this app in Airplane mode and be sure that the Dual Receiver is connected with the device and app you are using. When Dual is connected to the app, the button in the top navigation bar will show "Dual" in green letters, otherwise, the button will show "Apple" in red letters. You may have to restart your Apple device and perhaps restart the Dual Receiver once or twice to get a successful connection.

Disclaimer: Continued use of GPS running in the background can dramatically decrease battery life.


changes in version 2.3
  • Enlarged the Stop and Control buttons for easier touching
  • Touching the Control button pauses the odometers to permit time to estimate distance by tick marks to thousandths of mile or kilometer
  • Fixed bug to zero the odometers that may have previously been adjusted
  • Reduced speed at which distance estimates between GPS signals are received to 5.5 mph
  • Removed the buttons that reset the stopwatch to zero and cleared the tabular results. These functions are available when the Reset, New Rally button is touched and accepted
  • In Lap mode, the options to zero either of the odometers are removed and the Control button has been relabeled Lap Reset to indicate that the lower odometer will be reset to zero whenever that button is touched to begin each new lap
  • Various GUI changes
  • Several other bug fixes
changes in version 2.2
  • Major update to the Dual SDK version 2.2 that permits more effective data validation and Bluetooth connectivity. Thus, this app can now be used with iPads that do not have Cellular capability
  • Notable improvements in processing valid GPS data derived from Apple and Dual
  • Several GUI changes
changes in version 2.1
  • Updates to accommodate changes in iOS 13 and 14
  • Changes to accommodate formats of various iPhone and iPad displays
  • Minor code tweaks
  • Removed Facebook and Twitter links
changes in version 2.0
  • Added a continuously scrolling hundredth digit to each of the odometer displays
  • With the scrolling digit display, it is possible to interpolate distance to three decimal places. There are tick marks to indicate the second decimal place number centered on the display as well as increments of +0.0025, +0.0050, and +0.0075
  • The color of active odometer displays now vary with the direction of travel and there is a direction of travel label between the two odometers. These two features have been added to make it easier to determine the active odometer and the direction of travel at a glance
  • To facilitate estimating distance to three decimal places and to smooth the scrolling, the app estimates distance traveled between GPS distance inputs at up to 16 times per second when moving at speeds of 15 mph or greater
  • In order to allow more time for the navigator to interpolate distance, the odometer displays are paused for 5 seconds whenever the Control button is touched
  • Some other GUI enhancements and minor bug fixes

revised: July 26, 2022