Alti-Barometer V1 user's guide
AltiBarometer 256.png
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Welcome to Alti-Barometer!


This Apple Watch app runs independently of the iPhone and uses the watch’s barometric sensor to detect changes in barometric pressure due to weather changes, or to determine approximate altitude.

An important note on installation


Installing stand-alone Apple Watch apps is a little different than the more common companion apps. If you purchase an app directly on your Watch, it is installed and ready to use. But you can also purchase Watch apps in the App Store on your phone. This is where things get a little confusing. After you make the purchase, you then need to go to the App Store app on your Watch to download and install the app by following these steps:

1: Open the App Store app on your Watch:


2: Scroll all the way down and tap on "Account"


3: Tap on "Purchased" and scroll to "Alti-Barometer"


4: Tap the download icon to download and install the app:


Getting started


Please note that there are many factors that can affect pressure sensor readings and their interpretation, and that this app is exclusively for recreational and entertainment use only.  It is not intended for navigation or any situation where use of the app could compromise safety.


As is the case with dedicated altimeter-barometer watches, the app will operate in ether altimeter or barometer mode, but not both simultaneously.  


This is because barometers require an accurate altitude reference point and measure slow changes in air pressure over relatively long periods of time to determine weather-related pressure changes.  By contrast, an altimeter can respond to large and sudden changes in barometric pressure corresponding to changes in altitude.  The physical pressure sensor data is the same whether operating in altimeter or barometer mode, but how that data is interpreted and displayed will be specific to the mode of operation.


When installing the app, please accept the request to use your location; this is required for the app to function properly.  Per the app’s privacy statement at the bottom of this page, please know that your location information will only be used to enable the app’s operation and will not be shared or made available outside the app in any way.


The app launches into the “Mode” display.  Here, you can set English or metric units for operation, select the desired operating mode, and start or reset operation.


The default mode for operation is “Auto”.  In this setting, the app will make a determination based on the pressure data it processes to operate either as an altimeter or  barometer.  If the app senses slow and steady changes in pressure, it will display barometric pressure data.  If the app senses pressure changes greater or faster than can be caused by weather changes, the app will automatically switch to altimeter mode and display altitude information.


You can also select “Alti” or “Baro” to keep the app working in only that mode.


The app starts to record and process sensor data once you press “Start”.   You can delate all currently collected data and reset the app by touching and holding the “Hold to reset” button.  The reset requires you to hold the button for half a second to ensure that you don’t accidentally reset the app.


Pressing the “Start” button will automatically take you to the data display screen.  After some initialization, you will see the first recorded sample show up on the screen.  As the app continues to collect data in the background, this screen will fill with data points representing altitude or barometric pressure depending on the mode.


Data graph screens


The screen displaying collected data will display either the title “Baro-graph” or “Alti-graph” depending on the mode of operation.


You can examine any individual data point on the screen by twisting the Watch’s Digital Crown to scroll to the sample you want to highlight.


The data graph can display 36 data points at a time, and the app logs up to 144 total data points.  At the default logging interval of 5 minutes, the screen will show 3 hours of data.  You can use the Digital Crown to scroll through all of the logged data individually, and you can also tap either edge of the display to quickly advance or go backwards one screen at a time.  A single tap in the middle of the screen with hide or unhide the orange highlight cursor indicating which data point is being displayed, and a long press in the middle of the screen will take you back to the most recent sample.


In both altimeter and barometer modes, data points on the graph are color-coded to indicate direction (red: decreasing, green: increasing, gray: not changing), and the relative rate of change as indicated by the color intensity.  Although they appear similar, the displays for altimeter and barometer modes are optimized for each case as follows:


Baro-graph barometer graph screen


If the orange highlight cursor is active (e.g. you have scrolled the Digital Crown or tapped the screen), the data at the bottom of the screen will show the time the highlighted sample was recorded, the recorded pressure, and the reference altitude for the sample.   The trend arrow in the bottom right corner displays the pressure trend over the hour preceding the cursor position, and the trend one hour following the time at the cursor position.  If the cursor position is in a spot where there aren’t enough samples before or after the cursor position to make up a full hour of data, the corresponding portion of the trend arrow indicator will be hollow to indicate that there isn’t enough data to determine a trend at the current cursor position.  If the trend over the hour preceding or following the cursor position is flat, the arrow portion will be gray, if it’s increasing, green, and if decreasing, red.  Please note that positive or negative barometric trends are reported only for changes greater or less than 2 hectopascal (hPa) over 3 hours (i.e. 2/3 hPa per hour, or about 0.02 inches of mercury (inHg) per hour).  If the computed change is less than that threshold, the change arrow will be flat (gray) even though there may be a small but noticeable trend up or down.


If you dismiss the orange highlight cursor by tapping the middle of the screen, the data displayed on the bottom of the screen will show current barometric pressure, and the forecast based on that pressure.


The app uses the following values (in inHg) to calculate the forecast:


pressure < 29.0575   : Stormy

pressure < 29.4709   : Rainy

pressure < 29.8843   : Change

pressure < 30.2978   : Fair

pressure >= 30.2978 : Dry


Keep in mind that the barometer requires an accurate reference altitude to calculate an accurate barometric pressure reading.  If the current reference altitude and actual altitude are different, the forecast reading will not be accurate.


Alti-graph altimeter graph screen


If the orange highlight cursor is displayed (e.g. you have twisted the Digital Crown or tapped the screen to show the cursor), the corresponding data at the bottom of the screen will show the recorded altitude and time of day that the altitude was recorded.  The pressure trend arrow displays the altitude trend relative to the sample immediately before the cursor position, and the sample immediately after the cursor position.  If the cursor position is at either end of the graph, there won’t be a sample to use before or after the cursor position to determine an altitude trend.  To indicate this, the corresponding portion of the trend arrow indicator will be hollow to indicate that there isn’t enough data to display a trend.  If the trend relative to the cursor position is flat, the arrow portion will be gray, if it’s increasing, green, and if decreasing, red.


Positive or negative altitude trends are limited to changes greater than 20 feet per minute corresponding to intended recreational uses (e.g. hiking, running, or walking).  A change of less than 20 feet per minute will be interpreted as flat (e.g. not moving, or walking on level terrain).



Storm Alarm


The app includes a storm alarm feature.  If enabled, you will get a notification if the app detects a large barometric pressure drop (4 hPa, or .118 inHg) over a  2.5 hour time period continuously in barometer mode.  To receive storm alarm notifications, “Storm Alarm” must be enabled in the app on the Options screen, and notifications for the app must be enabled in the iPhone Watch Notifications settings for the Alti-Barometer app.


The storm alarm is also supported by the app’s complications using a red color for the status to denote an active storm alarm, or by using the ⚡️symbol in the large rectangular complications to indicate an active storm alarm.  The storm alarm will remain active in the complications for one hour after the storm alarm conditions have passed to help ensure that you’re aware of the alarm.




Both altimeter and barometer modes require calibration to display accurate interpretations of pressure data.  This is because barometric pressure decreases with increasing altitude, and the barometric sensor hardware is calibrated to sea level pressure.  Therefore, the altimeter requires knowing the reference height above or below sea level when starting operation to calculate the necessary adjustment for accurate display.  Because barometric pressure due to weather changes happen slowly over time, an altimeter reference point will produce reasonably accurate readings for hours.  Barometric pressure readings also require an accurate reference altitude, but will be inaccurate if the altitude changes.  Even climbing several of floors in a house will impact the accuracy of barometric pressure readings.


The app provides both manual and GPS-assisted reference altitude calibration as follows:


Manual calibration: For manual calibration, go to the Options screen, turn “Auto alt adjust” off, and and twist the Digital Crown to select your reference altitude (e.g. your current altitude) displayed in the altitude button at the button of the screen.  A long deep press of the altitude button will reset the reference altitude to 0.  You can also use the red and green quick-adjust buttons on either side of the altitude button to help you quickly get close to the desired reference altitude, and you can then use the Digital Crown to fine-tune the exact value you want.


Automatic calibration: If you turn “Auto alt adjust” on, the altitude button at the bottom of the screen will display “Getting GPS” to indicate that the app has started the process of trying to obtain a GPS location.  If successful, the button will display the GPS-acquired reference altitude.  If already in Auto mode, a long deep press on the altitude button will request a new GPS-based altitude calibration.  If the app is not able to obtain a GPS-based altitude due to reception difficulties, it will display the reference altitude in red.  In this case, the reference altitude used will be the most recent one.


Please note that obtaining a GPS-based reference may take a few minutes to locate the GPS satellites and to obtain high-accuracy data.




Alti-Barometer includes a Stats screen that displays total accumulated climb, descent, minimum barometric pressure maximum barometric pressure, minimum altitude, maximum altitude, and elapsed time since stats reset.  The stats are reset when the app is reset using the reset function on the home screen, but can also be reset independently by using the “Reset Stats” button on the Stats screen.  This can be handy when you want to track new stats (for example, maximum altitude hiking over that next ridge) but don’t want to reset the log of your entire hike.


Note that the climb and descent values will rarely ever match having arrived back at your original starting point.  There are a number of reasons for this including the fact that the barometric pressure is constantly changing along with the weather.  There’s no way to tell the difference between change in pressure due to change in altitude or change in weather, and the altimeter will interpret falling pressure due to weather as an increase in altitude, and vice versa.  Just a 0.1” drop in barometric pressure due to weather will show up as almost 90 feet of elevation gain, so it will look like you’re slowly climbing or descending just sitting in place as the weather changes.  With air pressure, it’s all relative!




The last screen on the app swiping right provides two options geared toward advanced users.


The "Min VSI" button allows you to change the threshold at which the app's complication displays the current VSI (vertical speed indicator) instead of the current altitude.  If you're more interested in displaying the current altitude on your watch face instead of small changes in altitude, you would set Min VSI to 200 feet as in the screen shot here.  Your vertical speed (rate of climb/descent) would then only be displayed on the watch face in the apps selected complication if the calculated rate of climb or descent was greater than 200 feet/minute.

You can use the Digital Crown to adjust this setting in single feet or meter increments, or larger steps down and up using the small red/green buttons on either side of the main VSI button (+/- 50 feet, or +/- 10 meters in metric mode).


The “Interval” button allows you to change the time interval between collected data samples in 15-second increments using the Digital Crown, or one-minute increments using the red/green speed buttons on either side.


The app default is 5 minutes which provides a good balance between having fine granularity of data and  being able to see a good chunk of time - 3 hours - per screen.  However, users may want more frequent samples to be able to see finer details of their climb or hike, or the progress of that line of thunderstorms coming through.


Note that any collected data will be reset if you change the sample interval, and that the scale at the bottom of the data graph will reflect the specified time interval.


Apple Watch Complications


The app includes support for a range of complications to provide “always-on” glanceable information.


The circular complication styles will display the appropriate altimeter or barometric trend direction arrows similar to the one displayed on the data graph display.  In barometer mode, the arrow shows the trend over the past two hours in addition to the forecast based on the last barometric pressure sample.  In altimeter mode, the arrow shows the trend over the previous two recorded altitude samples, and the trend based on the most recent recorded sample and current altitude.  This helps you know if you are climbing or descending without having to wait for the next recorded sample.  Altimeter mode also shows your current altitude, or alternatively, your rate of climb or descent if greater than 20 ft/min.  The “Graphic”-style complications will display the trend arrow using multiple colors.  Color support is more limited for non-graphic complication styles, and the color of entire trend arrow will represent the color corresponding to the most recent trend.


The two large rectangular complication styles display the maximum amount of information including trend, pressure or altitude, and forecast in barometer mode.  In altimeter mode, they will display climb or rate of descent if greater than 20 ft/min denoted as “VSI”, or vertical speed indicator, the standard nomenclature for avionics.  Although not intended or to be used as a substitute for aircraft instrumentation, the two large rectangular complications are very legible at a glance and can provide an additional reference in appropriate conditions.  In altimeter mode, the graphics portion of the rectangular graphics complication will be coded to display approximate oxygen levels at the calculated altitude, solid green designating sea level, orange as 8,000’ where some people may feel ill due to lack of oxygen, and red as 26,247’ corresponding to Mt. Everest’s “death zone”.  Note that altitude-based color coding is for reference only and not to be used for determining actual altitude-based oxygen levels.


Operational Notes

Hardware Requirements


This app requires an Apple Watch Series 3 or later.  Previous versions of the Watch do not include the necessary barometric pressure sensor in their hardware.

Battery life

This app has focused on battery life as a core principle.  It makes minimal use of GPS hardware with as little background processing as possible, and only updates the complications when needed to maintain functionality and usability.




The app does not record altitude readings while on the charger.  This is due to a quirk with how the Apple's WatchOS software for the pressure sensor works, specifically, the pressure sensor APIs stop sending updates to the app when the watch is on the charger.  I have requested that Apple change their API’s behavior and will update the app if and when Apple responds.


When on the charger, the Data graph screen will display “Paused…” in the data area at the bottom of the screen to indicate that it isn’t able to collect new data while connected to the charger.


When you remove the watch from the charger, it will pick up where it left off.  The app will interpolate the missing data, and will denote that the missing data has been artificially “filled in” by using purple dots on the data graph screens.