Thursday, June 7, 2018

Free Barometer App Designed for Mariners

Not many mariners realize that the most accurate barometer on the boat might be in their pocket... in their cellphones. There are not any native functions in iOS or Android phones that show the pressure reading, so you would not know it is there by just surfing around the functions of your phone.  It is not listed anywhere.  A third party app is needed to test that you have one.  There are many free versions of such apps, which brings up point one. Why do we make another one when there are so many already?

We want to let folks know about this tremendous resource, but then we are asked for an app recommendation, and the problem begins. Until now, the apps have all been too complicated. They want to do more, so they end up doing things like getting your elevation from the GPS, which even with a WAAS satellite connected is not accurate enough for barometer work adaptable to marine navigation.  In these days of remarkable weather services via model forecasts, we want the pressure accurate enough that we can tell if a surface analysis map is correct or not, so we can believe the forecast. In the tropics, we can also use accurate values of the pressure as a very powerful means of tropical storm forecasting.

Even worse than that, some apps get what is often called a "reference pressure" from the nearest airport, which are readily available online. Again, these maneuvers end up with pressures that are not accurate enough for our needs. There are even a few Andorid apps I have seen that do this without even saying what they are doing  In short, we could not find any app that we could recommend to our students to take advantage of this powerful, free resource.

Also only a few of the apps include a way to offset the sensor pressure, which, as good as the phone barometers are, they do, almost all, need some small adjustment (up to ± 2 mb or so) to be set right. So having an easy, transparent way to do that precisely is crucial.

Then the other common drawback is most apps do not have a way to store the data at all, let alone storing it with the time and location that we need.

So we have addressed these issues in the Starpath Marine Barometer App for iOS and Android.

Below are the four screens available to the user.

There is a rather detailed help file included with it to explain the features. You can set a sensor offset and we explain how to do that, and for sea level pressure you must manually enter the station elevation, ie how high is the phone above sea level when using it. In tidal waters we explain how to find the right elevation in the presence of tides.

You can store a pressure permanently, along with the time and your location.  Note that any app that uses the barometer has to be given access to Location Services in an iPhone, even if they are not using the location for anything.  Since there is no separate way in the phone to give access to the barometer sensor, this is the way Apple chooses to cover that permission.

We use the GPS sensor only for Lat, Lon, nothing else.  Also when you store the pressure it also records your sensor offset so you have a record of when that was done and under what conditions.

The only sort of bonus addition is we do add an option for averaging the pressure over the past 14 seconds. This covers the change in barometer elevation in a seaway. I will post a video soon that shows these functions in action—it's a mini workout; you do it on the stairs. This averaging can be shut off for use on land.

We do include the clear caution that the convenience of this accurate digital pressure from your phone—or any other electronic barometer—is not intended to replace a trusted aneroid barometer. All electronics are vulnerable to the rigors of the ocean environment. We compare a good mechanical instrument like the Fischer precision aneroid barometer to a sextant. It is a maritime investment that will remain dependable for generations.

We also note that the pressure data stored in the phone is intended to be transcribed to the ships printed logbook at your early convenience.

For more information and links to the app stores see Starpath Marine Barometer.


We have learned the Apple SE phones do not have a barometer. The barometer sensor seems to start with the ver 6 iPhones and newer. Also please note that not all tablets (Apple or Android) have a barometer, even newish ones. If you load the device and everything works except it shows no pressure then chances are your device has no barometer. This can usually be checked by a good inquiry about barometers in your model of phone.

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