GPS check

I had a dentist appointment on the eighth of March, so I took a train and, while I waited at Arapahoe at Village Center station, I compared my GPS coordinates as measured by Google Maps and the Physics Toolbar app. Both are easy. To get the coordinates from Google Maps, you just set a pin on the map where you are...poke your screen at the spot and hold it there until the teardrop shaped pin appears. Then you can read your coordinates in the search bar. Here's what I got.

Physics Toolbox is just as easy. The menu button at the upper left gives you a list of options. Choose GPS and the screen gives you the data. Here's mine.

So what I have from Googles Maps is:

Latitude: 39.600337

Longitude: -104.888418

and from Physics Toolbox:

Latitude: 39.600331

Longitude: -104.888407

That's pretty close, but how does it work out in distance?

There's a formula that gives you the distance between two points, specified by global coordinates on the Earth's surface. In the Astronomy LabBook, it looks like this:

I have it saved in my MC50 Programmable Calculator app so I don't have to crunch the numbers from scratch and my data has the same number of significant digits, so I can punch them right into the calculator. What I get is 0.000024 kilometers. That's 0.024 meters or 2.4 centimeters. Assuming that Google Maps is accurate, Physics Toolbar's stated precision of ± 5.36 meters is pretty conservative. 

Of course, the precision changes according to atmospheric conditions and the number of satellites in range (here we have 20 satellites.) 

GPS is important in any field science. It's easy to note your position in a field notebook when you have a pair of accurate global coordinates, and you should. At each observation you should note your position (if you can't get your coordinates, describe your position as best you can by giving the position in respect to some named landmark) and the date and time.

The smartphone has become a great companion for the field scientist. I never leave home without mine.