[Xastir] Xastir Digest, Vol 54, Issue 28

[Curt, WE7U]

On Sun, 23 May 2010, Guy Story KC5GOI wrote:

> I still think I am not getting the tie points close
> enough.  Based on what I learned on WinAPRS if you take the tie points and
> use them to draw a box, if it was more rectangular than square, the image
> would be altered and cause posits to not place correctly on the image file
> but are on the money with the Tiger files.

That sounds like a WinAPRS issue.  Xastir should work no matter
whether the "tiepoint box" is square or rectangular.  It only cares
that the points are accurately specified.  The bit about getting
them close to diagonal corners has to do with accuracy.  Xastir will
work if you specify two points very close together, but it will be
more accurate if the points are further apart.


> I am not completely to the upper left
> and lower right as I probably could be.

The exact corners don't matter, as long as you get as much vertical
spacing and as much horizontal spacing as you possible, dependent on
the tie-points you have to select from.  Xastir internally
translates the points you give it to corner pixels of the image.

It's too bad you have to resort to raster images.  I try to stick to
vector maps as much as possible to avoid these types of issues.  I
still use raster topo maps and sometimes aerial images, but stick to
the Tiger Shapefile-format maps for roads these days.

The datum shifts I see in my county are in the neighborhood of 90 to
100-something (105?) meters.  This shift is between NAD27 and
WGS84/NAD83 datums.

-- 
Curt, WE7U.                         <http://www.eskimo.com/~archer>
    APRS:  Where it's at!                    <http://www.xastir.org>
   Lotto:  A tax on people who are bad at math. - unknown
Windows:  Microsoft's tax on computer illiterates. - WE7U.
The world DOES revolve around me:  I picked the coordinate system!"