Actual Altimeter Instruction sheet (PDF format)
An important correction for the Electronic Distance Meter (EDM) is a correction for the density of the atmosphere. This correction factor is given in terms of PPM or parts-per-million. This is the equivalent to millimeters per kilometer. When surveying on the PSU campus, you will probably see a small correction – perhaps 8-10 ppm. Since you are measuring distances about 100m, the correction is less than 1 mm. This is small compared to the uncertainly in the standard distance measurement (3-5 mm ± 5 ppm). However, in many cases the correction can get large (40 - 50 ppm in eastern Oregon during the summer) an you may be surveying over distances of several hundred meters. At this point the correction factor becomes important. You should always be in the habit of entering your Temperature and Pressure correction.
The temperature correction is obtained using a standard thermometer. To get the pressure we use a barometer. Below are instructions for using the barometers we keep with the Total Stations.
Press either the left of right button. There is no off button for the barometers, they will turn themselves off after 30 seconds.
Press the left button. This will toggle between Altitude (ft or m) and Pressure (InHg or hPa)
Press and hold the right button for several seconds
The altimeter will now give readings in feet and inches of mercury or meters and hecto-Pascals (or milli-bars)
Questions? Comments? Contact: CruikshankK@pdx.edu
Page Last Updated: April 02, 2005