Very clunky, but if you have a consecutive numerical field in your table, you could add a new field which equals old field -1, then bring the table in twice and link by old field to new field. The rest should be easy.
Probably much easier in a database though. From: [email protected] [mailto:[email protected]] On Behalf Of Randal Hale Sent: Monday, 18 August 2014 6:43 AM To: [email protected]; [email protected] Subject: Re: [Qgis-user] Velocity Difference Calculation? The part of me that screams deadline thinks maybe you toss it into excel/calc and derive time then join that info back the distance portion. The minute I finished that sentence that might work for a few lines/distance - but you might have more than a few points/lines in the problem. To quote a person I once knew "That's a thinker". On 08/17/2014 06:34 PM, [email protected]<mailto:[email protected]> wrote: Yeah, I’ve got a distance reference that covers the amount of distance covered in the time from the last point. I’m just missing the way to actually do the calculation in the program. I cant see a way to compare a point to the one before it. Thanks! Dean Sent from Windows Mail From: Randal Hale<mailto:[email protected]> Sent: Sunday, August 17, 2014 3:22 PM To: [email protected]<mailto:[email protected]>, [email protected]<mailto:[email protected]> I can think of a couple of ways you might be able to do this - but considering I haven't done something like this before it's a guess. You can derive an interval in seconds I believe (referencing http://nathanw.net/2012/06/30/better-date-and-time-support-in-qgis-expressions-and-styles/) from the point data - I'm assuming you have date and time (or at least time). You could connect the points with a line (that would give you distance). I'm thinking this might be better accomplished in a database as opposed to a shapefile (assuming shapefile). ...and I think I've just given you the most abstract assumed non-answer ever. Lemme think on this one a bit more. Hopefully someone has a practical answer. Randy On 08/17/2014 05:57 PM, [email protected]<mailto:[email protected]> wrote: Hi all, I’m working on some point data that has been collected using a combine in an agricultural field. Trying to figure out a way to compare speed between the dropped points? I’m not sure how to start….can anyone help? Thanks Dean Sent from Windows Mail _______________________________________________ Qgis-user mailing list [email protected]<mailto:[email protected]> http://lists.osgeo.org/mailman/listinfo/qgis-user
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