Orientation system

[Rod Paterson]

> From grass-lists-owner%max.cecer.army.mil at munnari.oz.au Fri Dec  8 16:04:12 1
995
> Date: Thu, 7 Dec 1995 14:00:03 -0800
> I'm not so sure it's clockwise.  If I have perfectly horizontal text,
> it seems counterintuitive that "increase the angle by 30 degrees"
> should make the text run downhill.  If your users have a significant
> math background, they might expect you to follow _that_ convention
> (ccw=positive).
> 
> Much depends on your actual application--scripting language?  GUI to a
> cartographic tool?  Are the users map readers who don't remember
> algebra, or mathematicians who don't know how to shoot a bearing?
> 
> I think a better solution might be to get away from the terms
> plus/minus, and use east/west instead.
> 
> -- 
> Hal Mueller                             ManTech Environmental 
> EPA, NHEERL-Western Ecology Division    (541) 754-4490    Fax 754-4818
> 200 SW 35th Street                      halm at mail.cor.epa.gov
> Corvallis, Oregon 97333-4901  USA       http://www.zilker.net/~hal/
> 

 The terms to use that are least ambiguous are left and right as one
 travels along the vector azimuth.
 East and West won't do it when the vector runs E->W or W->E.

I am in favour of the following:

        If I wanted the symbol at +45 degrees I would expect it to be
        to the the right of the vector as I look along it in its direction
        of travel.
            ^       |
        ie. |       |
            |/     /|
            |       |
            |       v
        
        For a polygon stored clockwise and a symbol angle of +45
        all the symbols will be inside the polygon.
        For a polygon stored anticlockwise and a symbol angle of +45
        all the symbols will be outside the polygon.

        Does GRASS always store polygons in the same direction?

        The above strategy would conform to the way most earth
        scientists think about angles. ie 0-360 degrees clockwise
        However an applied mathematician using euclidean angles
        would no doubt do the opposite.

        Rod Paterson
        Aberfoyle Resources Ltd
        Level 31
        525 Collins Street
        Melbourne Vic., 3000

        Phone: 61-3-2706666
        Fax:   61-3-2706699
        Email: rgp at aberfoyle.oz.au