GPS surveying uses Global Positioning System technology to determine geographical and structural measurements as they pertain to various development projects, such as roads and construction. More accurate and less time consuming than traditional methods, GPS surveying encounters fewer distractions that can delay a development project, such as poor weather. The acronym PPM, standing for parts per million, expresses the accuracy of a relative orthometric height used in GPS-aided ground leveling.

Because the Earth is neither consistently round or smooth, scientists have devised a mathematical calculation that averages the planet’s highs and lows and assigns a theoretical sea level that's constant throughout the world, known as Earth’s ellipsoid. Ellipsoid height refers to the height of a given point or object above the ellipsoid, and its accuracy contributes to the PPM measurement expressed in the orthometric height.

Earth’s geoid assigns a hypothetical shape to the planet’s surface that runs through the actual shapes and contours formed by mountains, continents and bodies of water, regardless of their actual length or height. Unlike the ellipsoid, Earth’s geoid changes and forms an irregular shape. A geoid height is the length or height of a point or object above the mean sea level formed by the planet’s ellipsoid. Because the geoid changes in different parts of the world, the geoid height for the same object or point isn't the same at different spots on Earth. The geoid height’s accuracy lends itself to the overall accuracy of the orthometric height.

The orthometric height -- widely known as elevation -- expresses the distance between the point being measured and the Earth’s geoid. Because the geoid changes throughout the world, it's possible for the orthometric height to indicate elevations both above and below sea level. The geoid’s changing shape also makes it possible for you to stand on the beach of a major ocean and officially be standing at an elevation that's below sea level.

PPM expresses a standardized measurement of error -- in millimeters per 1,000 meters -- in relation to orthometric heights. For instance, an orthometric height that has a 2 PPM error rate would indicate an error in measurement equal to 2 millimeters per 1,000 meters traveled. So, if a mountain resort located 1,000 meters inland had a PPM of 2 millimeters, the orthometric height, or elevation, indicated would be accurate to within 2 millimeters.