Question about Tally on a list of lists

*To*: mathgroup at smc.vnet.net*Subject*: [mg99267] Question about Tally on a list of lists*From*: bleeckerj <bleeckerj at gmail.com>*Date*: Fri, 1 May 2009 05:23:55 -0400 (EDT)

Hi, So, I'm an on-and-off Mathematica user, just now looking at version 7, fussing with a challenge. I have data that represents geo tracks of latitude and longitude pairs. I want to turn this array of pairs into a heatmap covering the geography of the tracks. My first thought was to run Tally on the list and get counts for latitude/longitude pairs. That was an immediate fail because the pairs of data are mostly all different, of course. I thought if I could BinList things, piling them into "quadrants" of such-and-so dimensions (say, 1 kilometer square, or whatever) I could then count the number of bins. Alternatively, I'd even be happy to somehow reduce the precision of the numbers in the list, but after a few hours . I have not been able to figure out how to coax Mathematica to somehow truncate the numbers down to, say, 4 digits after the decimal point. This would create a kind of artificial Bin'ing of the data, which would be fine for my purposes. Removing the accuracy would allow me to effectively turn the data into a checkerboard of "hits." But, frankly, I'm not sure how to actually implement any of this, either the "truncating" or the Tally technique running over a list of lists with a SameQ sort of consideration to allow "close enough" to count in a Tally. I would be grateful for any pointers or suggestions as to how I might implement this. Julian Bleecker http://www.nearfuturelaboratory.com

**Follow-Ups**:**Re: Question about Tally on a list of lists***From:*Leonid Shifrin <lshifr@gmail.com>