Tru-Path

Row, row, row your boat gently up / down the stream???

Purpose: This blog is created to help readers a) better understand electronic compass [smartphone or rangefinder] residual azimuth deviation errors b) quantify the errors, c) model the errors, d) compensate for [correct] the errors, and e) influence the app vendors to apply the correction method within the affected smartphone app. Basically, we need to know (accurately) whether to go up / down the stream (path) we are traveling on.

Background:

This post will present the test results of the iPhone & GeoClino (geology) app operated at the Birmingham, AL test site within a strong magnetic/electromagnetic field. 

The Birmingham, AL (2-A) test site is set in an urban environment with strong magnetic/electromagnetic influences including:

• An electric power distribution station for eight (8) townhomes
• At least eight operating heating/cooling (heat pump) units of 3 to 4 ton capacity – aligned North/South within 50 feet of the test site (East side)
• An active highway – aligned North/South within 150 feet of the test site (West side)

Test Results

The data collection equipment and procedures used for this test (iPhone & GeoClino) were identical to those used at the Birmingham, AL test site for all prior posts.  Refer to the original post/page: “http://www.tru-path.org“

• True North (reference direction) was established based on the sun position relative to the test site’s geographic location on the date/time of the test – a correct, defendable, and independent reference direction.
• The iPhone compass was set to indicate azimuths relative to True North.
• The iPhone phone compass was re-calibrated prior to this test.
• The iPhone was mounted on a tripod equipped with an indexed compass ring – used to ensure the correct principal directions were achieved prior to taking the actual measurements. Therefore, no “sighting” errors were experienced during data capture operations.

Recall: Residual (after calibration) compass deviation error persists throughout the entire 360 degree range of measurement.  The following data table presents the collected azimuth data (yellow) and the associated deviation error (orange) for one iPhone orientation (horizontal). The compass deviation error data was collected in the same manner as all the other error data presented in this blog series.

The GeoClino app has done its best – yet residual compass azimuth deviation (error) persist.

The “modeled” deviation curve (derived from the residual deviation error) for the iPhone & GeoClino app is presented below.  The modeled residual azimuth deviation compensation curve is presented in two different formats to allow the reader to seriously consider the deviation error – as measured throughout the full 360 degree range of measurement.

The compensated deviation errors (remaining azimuth errors after compensation) for the iPhone & GeoClino app are depicted below.  Again, two display formats are provided to strengthen the perceived impact of the compensation method.

The predicted deviation measurement errors for the iPhone & GeoClino app are depicted in the following chart.  The predicted error curve is the negative of the deviation curve; and the compensation method proved quite effective.

Conclusions:

• The iPhone & GeoClino app indicate the following error ranges, within a 95% confidence limit: -4.7 < Horizontal < +10.5 degrees.
• The compensated iPhone & GeoClino app indicate the following remaining error ranges (after correction), within a 95% confidence limit: -1.2 < Horizontal < +1.2 degrees.
• The iPhone & GeoClino app is inaccurate and unreliable to determine compass azimuth values.
  • The GeoClino app gives different azimuth values when the measurements are taken in a counterclockwise direction as opposed to a clockwise direction.
  • In some instances, the azimuth readings were not repeatable until multiple attempts were made in a consistent direction (clockwise).
• While attempting to make successive azimuth readings – accessing the tripod index ring, the GeoClino app would shut down before the azimuth reading was successfully taken – an aggravating nuisance.
• The residual compass azimuth deviation error compensation method used in this test (research) is quite effective in compensating the error experienced.

Preview: In upcoming blog posts, we will deal with other Android and iPhone compass apps. As with previous tests involving the Android/iPhone compass apps, the same methods will be used to collect, analyze, and present the results. The next blog post will deal with the iPhone & Commander Compass Go app – if possible.