topic Re: plot directional GPS velocity vectors in ArcGIS Spatial Analyst Questions

plot directional GPS velocity vectors

JustinBrandt — Wed, 26 Jan 2011 18:34:51 GMT

Hello,

I have done some research on this subject, but can't find what I need. Back in 2008, someone on the old forum (forum.esri.com) posed this question but there are no replies. I'm hoping to restart a discussion.

I have a point shapefile with these attributes (about 90 or so entries):
[name, lat, lon, magnitude, degree_from_north, x-offset, y-offset]

These are GPS locations with horizontal velocities (magnitude and direction). So far I have them displayed as arrows pointing in the direction of movement (from rotation function in symbology) and I have the arrow size set proportionally to the magnitude.

I have two problems:

1) Is there a way to get the arrow's length to change instead of its size? i.e. the arrow just gets longer with larger magnitudes.

2) Is there a way to apply a custom offset for EACH symbol. Right now, the symbols are centered on the point, and I need the arrows to originate from the points.

Basically, something like this:
http://geon.unavco.org/unavco/GPSVelocityViewer.php

Thanks so much for any help.

Re: plot directional GPS velocity vectors

JustinBrandt — Thu, 27 Jan 2011 22:05:25 GMT

Okay, in talking with some local GISers and a lot of banging my head, I think I've found a solution to my own problem, so thought I'd post it in case there are others who have run into this (I know you're out there!)

I found the Wind Speed Tool here that others have had some success with. It turns out it wasn't working for me, but suffice to say all you need is a data file with site_name, lat, lon, magnitude, and direction from north (in degrees). I was able to use Excel to make my appropriate conversions.

Once I had the degrees_from_north and magnitude fields, I could simply use graduated symbols in the symbology tab to display the magnitudes (I used 8 classifications), then I applied a rotation based on my Degrees_From_North field. Then, to get the arrows to originate from the point (instead of being centered on the point), I only had to apply a custom symbol offset (from the symbol properties tab) for the 8 classifications I made, vs. the 90+ sites if I were to use proportional symbols.

It's not exactly what I wanted, since ideally I'd have a separate symbol size for each magnitude value, but that would require me to go through all 90 sites to set an offset for each symbol. Using the Graduated symbols saves me some time.

What would have been ideal is to be able to set a custom offset for each symbol based on a field calculation, similar to the rotation function. So actually, it was somewhat of a simple fix (except for the Excel magic) and gave me pretty much what I needed.

I hope this helps someone else.

Cheers,

Re: plot directional GPS velocity vectors

deleted-user-IR249IovB3CN — Thu, 02 Feb 2012 15:03:04 GMT

OK, I'm having a very similar problem working with sea surface currents vector data. Using MGET's Create Lines From Vector Component Rasters I have been able to create the correct vectors of magnitude and direction automatically. However I do not want the magnitude to determine the vector length, I want all vectors to be the same length and change the color based on magnitude. I'm not sure how the tool is creating the vectors in the first place and the tool code is not detailed enough to figure it out. Anyone have any guidance? Thanks.

Re: plot directional GPS velocity vectors

JustinBrandt — Wed, 08 Feb 2012 18:36:42 GMT

OK, I'm having a very similar problem working with sea surface currents vector data. Using MGET's Create Lines From Vector Component Rasters I have been able to create the correct vectors of magnitude and direction automatically. However I do not want the magnitude to determine the vector length, I want all vectors to be the same length and change the color based on magnitude. I'm not sure how the tool is creating the vectors in the first place and the tool code is not detailed enough to figure it out. Anyone have any guidance? Thanks.

I believe you can do that with a graduated symbol.

Go to the properties tab, click symbology, then under Quantities click graduated colors. You can then select the color ramp you want from the pull down menu and also adjust how many color breaks you want (i.e. group several magnitudes into a single color).

Hope that helps.

[ATTACH=CONFIG]11803[/ATTACH]

Re: plot directional GPS velocity vectors

deleted-user-IR249IovB3CN — Mon, 13 Feb 2012 20:25:30 GMT

Yeah, I was able to define the colors of the vectors as graduated symbols using the current magnitude as the attribute field to define colors. The issue was also related to this particular tool generating the vector (or arrow) lengths using the magnitude. For example in fast moving currents the vectors were long and in slower moving currents the arrows were short or almost non-existent. The MGET team will be adding an option to export these data as vectors of uniform length. Until then the work around is to create a point feature class using the Arcpy function FeatureVerticesToPoints_management to create the points from the vector starting vertex. Then I was able to symbolize the points as character marker symbols to create the desired arrow symbology.