My first hands on experience with GPS was on a Soviet Seismic ship called the Akademik Kreps, where the company I worked for provided the navigation system. Seismic surveyors explore for oil on land and offshore by sending an energy source into the ground, then recording the energy returns on a chart, and mapping this chart to the real world.
At the time we could only use GPS twice a day for about 2 hours each instance. We were switching between GPS, Transit satellite and dead reckoning. We were too far from shore to use ground based radio navigation systems or DGPS and we were working in the Russian Chukchi and East Siberian seas without ground support.
With this coarse quality of positioning finding your way back to within 100 metres of a shot point in the middle of the ocean was considered close enough. However, we were still mindful of where the GPS antenna was placed with respect to the hydrophone streamer and the air guns generating the seismic energy, and took the time to measure those offsets to within the nearest metre.
Today we only use RTK quality GPS receivers on High Precision systems. Not that we need that much precision for horizontal positioning but to get the most accurate elevation possible. To do this we need to know where the GPS antennas are located with respect to the body and tracks of the machine. The centerline of the machine and the boom joint are good horizontal reference points in the case of a shovel.
The antenna precision with respect to the base station is 1-2 cm. We want to match this precision, or better, when taking our offset measurements from the body frame to the GPS antennas. The most reliable way to do this is to use a total station (the electrical/optical instrument used in modern surveying). This may seem like overkill to some, but remember, that our errors grow the further we get away from the GPS antenna so we should minimize measurement error whenever possible.
For smaller equipment such as loaders and dozers (relatively speaking of course, all mining equipment is huge), where we are measuring the antennas with respect to the corners or centerline of the cab, offset measurements can be done with a tape measure. However, I still prefer a total station. The total station makes it easier to get the measurement from the antenna, to track level, and to the edge of the bucket or blade.
Referring back to the first two paragraphs of this article, can you guess what year it was when we were seismic surveying on the Kreps?