In the recent years of budgetary cutbacks and cost cutting measures, one of the common questions raised revolves around why a state or local agency such as the Property Appraiser, Police/Fire Department or Environmental Department should spend hundreds of thousands of dollars for imagery when there appears to be an abundance of freely available imagery on the “web”.  This question has been asked of me on multiple occasions from multiple sources in my agency, as well as, from other agencies.  Hence, this article is of relevance to discussions, perhaps, state-wide.

The well written article by Mr. Kalinski highlights the advantages and disadvantages of using “Commercial Off-the-Shelf” (COTS) imagery available from Google, Bing, etc.  The crux of the matter resolves to a few basic issues; (1) you get what you pay for, and (2) not all imagery is the same. 

In a table (reproduced below) Mr. Kalinski points to the availability of metadata, resolution, currency and cost as the major factors that should influence the end-users decision.  He ends the article by reminding the end-user that there is no warrantee of usability with “free” imagery… caveat emptor! (my words)
 

This topic has arisen within the state, and in the Fall of 2012, several of the Water Management Districts and the Florida Department of Transportation engaged in an exercise to determine the positional accuracy of COTS imagery.  I constructed a set of guidelines and we independently applied them to Google and Bing images for our respective Districts.  In general, we used a photo identifiable, surveyed points that had been previously used for ortho-imagery horizontal accuracy testing to test the COTS images.  While there is some variation, most likely resulting from different image years, the results were fairly consistent (and shown below).
   

In general, we found that Bing images, as viewed through a then-available free  ESRI Map service, had greater horizontal accuracy than Google.  In fact, Bing image horizontal accuracy is very close to the horizontal accuracy specification for state-wide 30cm ortho-imagery.  So… as suggested by Mr. Kalinski, if we were interested in locating a favorite fishing spot, the Bing image would be great.  However, if we are using the imagery in a court of law, we have a very different end-use.