Home > Public Domain Data > Be Critical of Data–Even when it is your own!

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

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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