Topic: what altitude to use for calculating average sea level pressure???

[gurmeet.athwal]

Hi, I am  Gurmeet, from India. I am trying to build a tiny Weather station with arduino, I don't know much about weather forecasting.
now I am stuck at calculating "average sea level pressure", as it requires current altitude for calculation. what altitude information should I use? barometric altitude or GPS altitude? ? as both of these are diffrent, the barometric altitude at my place is 288Meters and GPS altitude is 182Meters. what is the correct way to calculate average sea level pressure??
please help. 

[Jáchym]

Hi Gurmeet,

if I remember correctly, the sea-level conversion should be done using the barometric pressure, maybe if someone could also confirm this please.

[gurmeet.athwal]

hi Jáchym,
thanks for the reply.
yes you are right, it should be done using barometric pressure.
I am trying to calculate sea-level pressure using the formula at http://keisan.casio.com/exec/system/1224575267
as it requires the current barometric pressure at the station and the altitude of the station I need to know whether GPS altitude should be used or Barometric altitude should be used??
using Barometric Altitude I get Sea level pressure at 1,016.52 hPa,
using GPS Altitude I get Sea level pressure at 1,004.46 hPa.

[nincehelser]

What you really need is your elevation as determined by an actual survey.

[Jáchym]

hi Jáchym,
thanks for the reply.
yes you are right, it should be done using barometric pressure.
I am trying to calculate sea-level pressure using the formula at http://keisan.casio.com/exec/system/1224575267
as it requires the current barometric pressure at the station and the altitude of the station I need to know whether GPS altitude should be used or Barometric altitude should be used??
using Barometric Altitude I get Sea level pressure at 1,016.52 hPa,
using GPS Altitude I get Sea level pressure at 1,004.46 hPa.

Yes, I understand and I think you should be using the barometric pressure not the GPS one

[gurmeet.athwal]

thank you.
will be using the barometric one..

[nincehelser]

hi Jáchym,
thanks for the reply.
yes you are right, it should be done using barometric pressure.
I am trying to calculate sea-level pressure using the formula at http://keisan.casio.com/exec/system/1224575267
as it requires the current barometric pressure at the station and the altitude of the station I need to know whether GPS altitude should be used or Barometric altitude should be used??
using Barometric Altitude I get Sea level pressure at 1,016.52 hPa,
using GPS Altitude I get Sea level pressure at 1,004.46 hPa.

Yes, I understand and I think you should be using the barometric pressure not the GPS one

That sees a little circular.  The inputs should be the station pressure, altitude, and temperature. (based on the formula he is using)

The best altitude/elevation to use would be one from a survey or topo-map.  GPS would be the next best alternative.

Using the altitude from the same baro sensor reading you are trying to adjust to sea-level doesn't make much sense.

[Mattk]

Barometric pressure is related to mean sea level based on an orthometric height which can be determined from GPS (ellipsoid) height using a separation distance. This separation distance (which can be positive or negative) varies depending on your location but can be computed from the various country or world geoid models available.   

[Old Tele man]

What's the elevation above sea level?

The altitude at my house is 2330-feet (or 710 meters), which is, from the altitude-tables, about 0.918-0.919 of sea level pressure:

• 27.47"Hg = 0.918 x 29.92"Hg
• 930mb = 0.918 x 1013.25mb

Or, you can calculate the value yourself:

P(sta) = P(sealevel)*[ (288-0.0065*h(m))/288 ]^5.2561

930.8mb = 1013.25mb*[ (288-0.0065*(710m))/288 ]^5.2561

[gurmeet.athwal]

hi Jáchym,
thanks for the reply.
yes you are right, it should be done using barometric pressure.
I am trying to calculate sea-level pressure using the formula at http://keisan.casio.com/exec/system/1224575267
as it requires the current barometric pressure at the station and the altitude of the station I need to know whether GPS altitude should be used or Barometric altitude should be used??
using Barometric Altitude I get Sea level pressure at 1,016.52 hPa,
using GPS Altitude I get Sea level pressure at 1,004.46 hPa.

Yes, I understand and I think you should be using the barometric pressure not the GPS one

That sees a little circular.  The inputs should be the station pressure, altitude, and temperature. (based on the formula he is using)

The best altitude/elevation to use would be one from a survey or topo-map.  GPS would be the next best alternative.

Using the altitude from the same baro sensor reading you are trying to adjust to sea-level doesn't make much sense.

The barometric altitude of 288 meters is the altitude I searched on google for my city (Rampur, Uttar Pradesh, India)
The GPS altitude 182 meters is what I get on my phone GPS.

Using 288 meter the MSL is closer to standard sea level pressure.
And
Using 182 meters the MSL is closer to nearest weather station which is 190 km away.

[nincehelser]

hi Jáchym,
thanks for the reply.
yes you are right, it should be done using barometric pressure.
I am trying to calculate sea-level pressure using the formula at http://keisan.casio.com/exec/system/1224575267
as it requires the current barometric pressure at the station and the altitude of the station I need to know whether GPS altitude should be used or Barometric altitude should be used??
using Barometric Altitude I get Sea level pressure at 1,016.52 hPa,
using GPS Altitude I get Sea level pressure at 1,004.46 hPa.

Yes, I understand and I think you should be using the barometric pressure not the GPS one

That sees a little circular.  The inputs should be the station pressure, altitude, and temperature. (based on the formula he is using)

The best altitude/elevation to use would be one from a survey or topo-map.  GPS would be the next best alternative.

Using the altitude from the same baro sensor reading you are trying to adjust to sea-level doesn't make much sense.

The barometric altitude of 288 meters is the altitude I searched on google for my city (Rampur, Uttar Pradesh, India)
The GPS altitude 182 meters is what I get on my phone GPS.

Using 288 meter the MSL is closer to standard sea level pressure.
And
Using 182 meters the MSL is closer to nearest weather station which is 190 km away.

I guess we're having a terms barrier.

According to wikipedia, your city elevation is 288 meters.  I'm not sure why you're calling this a "barometric altitude".  It's something you should get from a survey or topo map, not from a barometer.

[gurmeet.athwal]

Ok..
So should I use 288?

[nincehelser]

Ok..
So should I use 288?

It's a reasonable number for your city, but I've no clue what your local terrain is like.  If it's pretty flat, it might be good enough.  If hilly or mountainous, you might want to fine tune it with a topo map.

[gurmeet.athwal]

Ok, Yes is is completely  flat.
Thank you..