Geometry of the Sphere - Spatial Mathematics: Theory and Practice through Mapping - page 21

such situations. The latitude and longitude readings will also fluctuate more
rapidly in such places.
Place your hand across the top of the phone. Does your accuracy degrade,
and by how much? Depending on your local conditions, your signal should
degrade from a few meters to tens or even hundreds of meters or stop work-
ing entirely. Start walking. How accurate is your position relative to the base
map? Your recorded position on the base map should fall onto the opposite
side of a nearby building, or in the next block, or on the other side of town.
Go inside. Can your phone still determine its position? If so, does its accuracy
degrade in any way, and by how much? When you first enter a building, your
position should be captured for a while. If you have many nearby windows,
or if you are under a certain kind of roof, your smartphone may be able to
detect your position for a while, or even indefinitely, though with a reduced
accuracy and precision as when you were outside.
To analyze the positions that you collect on your smartphone in a more
rigorous way, collect a track on your phone that you can map later using
an online GIS. On an iPhone, use Motion X GPS or an equivalent tool to
capture coordinates as a track. On an Android, use MyTracks as an equiva-
lent app to accomplish the same thing. Start your collection while inside a
building, and walk outside while continuing to collect tracks. As you did in
the previous activity, cover part of the phone with your hand, and access
the Motion X GPS or MyTracks screen while doing so. Can you determine
how many GPS satellites you are blocking? Depending on your local envi-
ronment, and the version of the smartphone app that you are using, you
should be able to see that you are blocking at least one and possibly all of
the GPS satellites.
Save your track as a GPX file. A GPX file is a “GPS Exchange Format” file
that contains coordinates that you collect in the field. Email the GPX track
to yourself using the function in Motion X GPS or MyTracks. Retrieve your
email and save your GPX file to your local computer. Or, alternatively, save
your track as a KML file and email it to yourself. A KML file is a Keyhole
Markup Language file in wide use for Earth browsers such as Google Earth
and other tools.
Now that you have collected your track, you are ready to map it. To do so,
access ArcGIS Online ( www.arcgis.com/home ) and make a new map. Use the
Add button to find your GPX or KML file stored on your local computer or
online. Once your track is mapped, change the symbols of your track points
if desired, to map your tracks based on elevation or time. Change your base
map to a satellite image so that you can determine the accuracy of your track
based on the image beneath your points. Remember that the image base map
is not perfect either—it was created using a series of mathematical algorithms
and contains distortion and error. Use the measure tool to determine the dis-
tance between where you know you walked on the ground, using the image
to verify, versus where the track points are located.