# Options for precise relative position calculations

I'm investigating an industrial application that has a requirement to calculate with precision the relative distance on the ground between two outdoor points in the following scenario:

1. The first point will vary in height from 10m to 150m off the ground

2. The second point will always be around 3m - 5m off the ground

3. The points will be separated by 0m to 3m on the ground plane

4. I'm only interested in the separation on the ground, not the vertical separation

5. The precision should be within 10cm and I don't care about the absolute position accuracy.

6. Both points could be slowly moving around any axis before stabilising.

Looking into options available, I'm thinking standard GPS should be able to give me this type of precision if I compute the relative differences in position, which should hopefully compensate for the absolute positioning errors that GPS normally has, so I can avoid the complexity / cost of DGPS system.

I'm unsure if the differences in vertical height between the 2 points will impact the precision, and I also assume I have to use the a quality GPS receiver of the same type for both positions (to remove any differences caused by the receiver).

If this won't work, do I have any other options for this use-case (eg. combining an IMU with the GPS data, DGPS)?

• Surveying equipment? A measuring tape? This thing? I would google 'sub 10cm GNSS' or 'sub 10cm GPS'. Also this. – jbalk Sep 10 '19 at 0:32
• Hi, can't use measuring tape due to the significant differences in height. My question is mostly about the possibilities of using 2 quality GPS receivers that are transmitting their location to a third unit that can real time calculate with precision the relative differences in position. I can design my own electronics / software for this solution. – deandob Sep 10 '19 at 0:57
• Good question! Although good GPS devices are accurate to a few feet, is the horizontal inaccuracy the same magnitude and direction for two devices in close proximity at the same time? Accuracy is dependent on the quality of the unit for sure but also how many satellites are above the horizon and their dispersion across the sky. Perhaps do and experiment with using two cheap cell phones (not good GPS units at all, be sure to turn off location service) a reasonable time stationary then moving one then the other whilst logging location and see what you come up with. – Michael Stimson Sep 10 '19 at 1:23
• Good feedback, thanks. I tried a couple of andriod GPS test apps that displayed the precision required (7 decimal places for 10cm precision) and moving several metres was required before they registered a change. So I assume I need a higher quality receiver (and antenna) to get better resolution / precision. – deandob Sep 10 '19 at 2:53
• You can get a better antenna for an Android device but I've been told it takes a bit of mucking around in the OS to switch from the external to the add on; be sure to turn off location service (AKA high accuracy) which uses cell phone towers to triangulate your location (when available) so that you're testing the GPS of the device. This often comes as a bit of a surprise to city users heading out into the wild where their accuracy goes from fairly reliable to absolutely horrible as soon as they are out of range of 3 or more towers. – Michael Stimson Sep 10 '19 at 3:07