Compensating for Altitude in Levelogger Data

Barometric Compensation Review

As you know, Leveloggers measure total or absolute pressure. When submerged, they record a combination of atmospheric pressure and water pressure above their sensor. Because the intent of using Leveloggers is to obtain fluctuations in water pressure only, their data must be compensated for atmospheric effects.

In order to achieve this, atmospheric (barometric) pressure must be subtracted from the Levelogger readings. The easiest way to barometrically compensate your Levelogger data is to use a Barologger that simultaneously records this barometric data.

Correcting Levelogger and Barologger Data

Atmospheric pressure can be considered the total weight of air above an object at any elevation. As you go higher in the atmosphere, there is less air above to weigh down the object. This translates to decreasing pressure as you go higher in the atmosphere.

So, what does this mean when barometrically compensating your Levelogger data?

Water column equivalent pressure decreases with altitude at a rate of approximately 1.21:1000 feet or meters (altitude/826) in the lower atmosphere below 16,400 ft. (5000 m). Therefore, if the Barologger that you are using to compensate your Levelogger is at a different elevation, there will be different atmospheric pressures associated with their locations.

Generally, you can use a Barologger to compensate Leveloggers within an elevation change of 1000 ft. (300 m) (and within a 20 mile (30 km) radius). If your Barologger and Levelogger are further than 1000 ft. (300 m) apart in height, then you have to consider correcting for this difference to get the most accurately compensated data.

Depending on the accuracy of data that you require, you may always want to consider accounting for altitude differences.

If your Barologger is deployed at a higher elevation than your Levelogger, then your Barologger will be recording a lower atmospheric pressure than what the Levelogger site is experiencing at static water level (atmospheric pressure acts on the static water level above the Levelogger). Before you can accurately compensate your Levelogger data, you need to add this difference in pressure to your Barologger data. The same goes when your Levelogger is deployed higher than your Barologger – you need to add the difference to your Levelogger data.

To simplify things, correcting for elevation differences can be done right in the Levelogger Software. When using a Levelogger 5 series datalogger, correcting for elevation is done post data collection. When compensating your data using the Levelogger Software Data Wizard, you can select the Advanced option, which allows you to enter the elevations at which your Levelogger(s) and Barologger are deployed.

You can enter elevations between -1000 ft. below sea level and 16,400 ft. (or -300 m and 5000 m) above sea level. The readings will then be automatically compensated for elevation differences. This can be done separately, or simultaneously with Barometric Compensation in the Advanced option of the Data Wizard.

Manual Data Correction

However, if you are manually correcting your data using a spreadsheet program, you will have to do the calculations yourself. Here’s an example:

Your Levelogger is recording at 500 ft. AMSL (above mean sea level). Your Barologger is recording at 2000 ft. AMSL. At the 12:00 am timestamp, your Levelogger recorded 40 ft. and your Barologger recorded 99.9 kPa.

First, determine the difference in elevations: 2000 ft. – 500 ft. = 1500 ft.

Use this to determine the barometric pressure differential:
elevation / 826 = barometric pressure differential
1500 ft. / 826 = 1.816 ft.

Since the Barologger is above the Levelogger, you will add 1.816 ft. to your Barologger readings, before proceeding with barometric compensation.

Note: If it were the opposite, and your Levelogger was at 2000 ft., and your Barologger was at 500 ft., you would add 1.816 ft. to your Levelogger readings.

To complete the barometric compensation, first, you have to ensure the Levelogger and Barologger data are in the same units.

Convert your Barologger reading from kPa to ft.:
99.9 kPa x ft. water column equivalent conversion factor = ft. H20
99.9 kPa x 0.335 = 33.467 ft.

Note: for water column equivalent conversions for other units, please see the Levelogger User Guide.

Now add the pressure differential to the Barologger reading: 33.467 ft. + 1.816 ft. = 35.283 ft.

Now subtract the corrected Barologger reading from the Levelogger reading:
40 ft. – 35.283 ft. = 4.717 ft.
Therefore, the true height of water above the Levelogger sensor was 4.717 ft.

Correcting Weather Station Data

The same methodology is a bit trickier if you did not use a Barologger to collect atmospheric pressure data. If you did not use a Barologger to collect readings for compensation, you will have to obtain them from another source – most often, this will be from a weather station.

It is important to remember that weather station barometric data will often contain an offset or normalization. Most weather stations normalize barometric pressure to sea level. In this case, you must determine the difference between the Levelogger’s elevation and sea level to get the true amount of pressure acting on static water level.

Using the example of the Levelogger at 500 ft. AMSL and the weather station data normalized to sea level, the difference in elevation is: 500 ft. – 0 ft. (sea level) = 500 ft.

Use this to determine the barometric pressure differential:
elevation / 826 = barometric pressure differential
500 ft. / 826 = 0.6053 ft.

Since the Levelogger is above the weather station in elevation, you will add 0.6053 ft. to the Levelogger readings, before proceeding with barometric compensation.

Note: remember, weather station data may also include an offset, in addition to normalization, that you may have to correct for.

Using Levelogger Gold and Barologger Gold data? Refer to the Levelogger Software Version 3.4.1 User Guide for more information.