The Gumstix Pi Pre-GO Project Chapter 3

There was a nice break in the weather yesterday and I managed to get outside to take some readings.  I set up on an exposed hilltop with my laptop, my Pre-GO-to-go and my smartphone.  I was hoping that I might find some kind of survey marker with exact GPS coordinates at the base of the flag pole, but no such luck! I fired up my Pi, gave it a few minutes to warm up, and connected it and my laptop to my phone's WiFi hotspot.

With no guarantee of clear weather, I decided to do a quick test and grab four different data snapshots over a 15 minute window.  I'll formulate some more informative tests for when the weather is better.  For now, I just want to see how accurate it is.

The PPP in Pre-GO PPP stands for Precise Point Positioning.  PPP provides centi- to decimeter-level accuracy so, even if Google doesn't drop its pin directly on the flagpole, my data points should be very close together.  I have to assume that Google Maps co-ordinates are going to be somewhat skewed.  Without an accurate geodetic survey marker, I can't be certain.

When I got back to the office, I dropped some pins on the map and here's what I came up with:

The outlier is my initial result.  The tight grouping south west of it are the three other results I recorded.  They sit about 1.5m away from the outlier but are all within 12cm of each other.

I found this blog post that confirms my assumption about Google's geodetic accuracy.  If you compare my Maps screen shot to his:

It looks like the skew is about equivalent.  I trust my results over Google's now.

So I still have no concrete baseline measure of accuracy.  I really want to know if my horizontal co-ordinates, when HDOP (horizontal dilution of precision) is less than 1, are accurate at the decimeter level.

To that end, my colleague sent me a link to waymarking.com, where I found a survey marker nearby.  GPS enthusiasts have, thankfully, taken the time to accurately measure its geographic position.  OK great, but I have been rained out.  I guess the next test is going to have to wait awhile.  While we wait, let's see if the Pre-GO works with the BeagleBone Black and the Gumstix Rover cape.