Altitude of a plant location is very important in terms of designing of your vacuum systems, identifying boiling points for correct calculations & design of distillation system, boil-off systems etc.
It is also important in terms of correct sizing of condensers, reboilers due to change in enthalpies, significant change in LMTD will cause errors in design if percent variation is large specially when temperature difference are lower.
Therefore, it should be identified correctly.
Though the best method is to measure it at site location using barometers, however this is not always possible due to absence of such meters. The major problem in it is that normally designers are actually away from the location.
So here are two quick methods to understand the actual barometric pressure.
Use any vacuum pump which is reliable & relatively new. if you are installing a new pump or have recently installed a new pump then run it on shut off condition with a pressure transmitter in the suction line. The shut off reading will practically be equal to the barometric pressure at site.
But you need to be sure in this case that pump should be designed for 1 torr or lesser shutoff pressure. if it is designed for higher shut off pressure, you need to subtract the design shut off from actual reading of pressure transmitter during test mentioned above.
So, if pump is designed for shut off pressure of 720 Torr and pressure reading during shut off test is 10 torr then barometric pressure will be 730 Torr at that site.
Another method is use of google map service. For this you need to download the file altitude.html from the link Here.
Now you need to just identify your plant location and you will quickly come to know the altitude of your plant from MSL (Mean Sea Level).