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Be Critical of Data–Even when it is your own!

February 3, 2013 2 comments

A theme running throughout our book The GIS Guide to Public Domain Data is to be critical of the data that you are using–even data that you are creating.  Thanks to mobile technologies, anyone can create spatial data, even from a smartphone, and upload it into the GIS cloud for anyone to use.  This has led to incredibly useful collaborations such as Open Street Map, but this ease of data creation means that caution must be employed more than ever before.

For example, analyze a map that I created ) using Motion X GPS on an iPhone and mapped using ArcGIS Online.  It is shown below, or you can interact with the original map if you prefer.  To do so, access www.arcgis.com/home (ArcGIS Online) and search for the map entitled “Kendrick Reservoir Motion X GPS Track” or go directly to http://bit.ly/Rx2qVp.  Open the map.  This map shows a track that I collected around Kendrick Reservoir in Colorado USA.  This map was symbolized on the time of GPS collection, from yellow to gradually darker blue dots as time passed.

GPS track around Kendrick Reservoir

GPS track around Kendrick Reservoir

Note the components of the track to the northwest of the reservoir. These pieces were generated when the smartphone was just turned on and the track first began, indicated by their yellow color.  They are erroneous segments and track points.  Notice how the track cuts across the terrain and does not follow city streets or sidewalks.  Change the base map to a satellite image.  Cutting across lots would not have been possible on foot given the fences and houses obstructing the path. When I first turned on the smartphone, not many GPS satellites were in view of the phone.  As I kept walking and remained outside, the phone recorded a greater number of GPS satellites, and as the number of satellites increased, the triangulation was enhanced, and the positional accuracy improved until the track points mapped closely represented my true position on the Earth’s surface.

Use the distance tool in ArcGIS Online to answer the following question: How far were the farthest erroneous pieces from the lake? Although it depends on where you measure from, some of the farthest erroneous pieces were 600 meters from the lake.  Click on each dot to access the date and time each track point was collected.  How long did the erroneous components last?  Again, it depends on which points you select, but the erroneous components lasted about 10 minutes.   At what time did the erroneous track begin correctly following my walk around the lake? This occurred at 11:12 a.m. on the day of the walk.

This simple example points to the serious concern about the consequences of using data without being critical of its source, spatial accuracy, precision, lineage, date, collection scale, methods of collection, and other considerations.  Be critical of the data, even when it is your own!

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Covering Our Tracks

September 17, 2012 Leave a comment

In our not-so-distant, non-digital past, good guys pursued bad guys with the aid of a faithful hound or trusty local guide. The trail of said bad guys could always be followed from the footprints they left or the vegetation they displaced, subtle impressions in the natural environment that lingered for a short while before they were erased by wind, rain or the occasional foraging coyote.

Nowadays, those of us who never leave home without our mobile or cell phone, leave a potentially more sinister and permanent record of our travels.

James Ball and Craig Timberg recently wrote an article for the Washington Post about the ongoing legal debate in the USA about the use of GPS data in criminal cases (GPS technology finding its way into court). In two unrelated cases, cellphone location data were used to help secure arrests, and the use of that data was sanctioned by the courts. The individuals involved did not have a “…reasonable expectation of privacy” when it came to their cell phone data and it could be collected, without a search warrant. Fearing the potential for abuse as increasingly sophisticated tracking technologies become available, legal challenges to use cell phone location data in this manner were immediately launched. The debate looks set to make it all the way to the Supreme Court.

It’s not just GPS-enabled phones that are involved. Phones without GPS devices, and their owners, can be tracked as they move between signal towers. How many phone users are aware of this and how many know who collects and has access to that information? If we knew more, would we modify our cell phone usage or would we trust the data collectors not to abuse our private location information?