We are looking for the average altitude of where you will be hunting/shooting in feet (FT) above sea level. You turret will remain accurate within 2,000 ft above and below the entered value (i.e. you have 4,000 FT window to work with). In terms of environmental factors, altitude and temperature are going to have the largest impact on the accuracy of your turret. Therefore, if you plan to use your firearm in multiple locations with large variances in altitude, we recommend purchasing multiple turrets with different altitude values to ensure turret accuracy. The easiest way to find the altitude of your location is a simple google search (e.g. altitude of 'my location').