[Qgis-user] Velocity Difference Calculation?

[Damien Stephens]

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: qgis-user-bounces at lists.osgeo.org [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of Randal Hale
Sent: Monday, 18 August 2014 6:43 AM
To: dean.walker151 at gmail.com; qgis-user at lists.osgeo.org
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, dean.walker151 at gmail.com<mailto:dean.walker151 at gmail.com> 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

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From: Randal Hale<mailto:rjhale at northrivergeographic.com>
Sent: ‎Sunday‎, ‎August‎ ‎17‎, ‎2014 ‎3‎:‎22‎ ‎PM
To: dean.walker151 at gmail.com<mailto:dean.walker151 at gmail.com>, qgis-user at lists.osgeo.org<mailto:qgis-user at lists.osgeo.org>

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, dean.walker151 at gmail.com<mailto:dean.walker151 at gmail.com> 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


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