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[aprssig] Map Symbol Placement

Curt, WE7U archer at eskimo.com
Thu Apr 19 15:47:29 UTC 2007


On Thu, 19 Apr 2007, 'Scott Miller' wrote:

> Of course, maps aren't going to be linear if they're flat, and the Earth
> isn't perfectly spherical.  To do it right you need to know what projection
> the map uses.  Xastir is the only APRS program that I'm pretty sure knows
> about map projections, and I think it uses an external library to handle
> them.

Xastir does know about map projections, via libproj, but we don't
use it for very much currently.  We assume a flat, rectangular map
for most image files we handle, the only exception to this is USGS
DRG's, maybe DOQQ's as well.

When we need to use maps in various projections with Xastir we warp
them using other tools ahead-of-time.  Xastir itself doesn't have to
know about it.

Some map programs are capable of using many tie-points to warp the
image to a flat-map representation.  We usually use "gdalwarp" to
process maps for use with Xastir.  It's free.  From the manpage:

"SYNOPSIS
  gdalwarp
    [-s_srs srs_def] [-t_srs srs_def] [-order n] [-et err_threshold]
    [-te xmin ymin xmax ymax] [-tr xres yres] [-ts width height]
    [-wo "NAME=VALUE"] [-ot Byte/Int16/...] [-wt Byte/Int16]
    [-srcnodata value [value...]] [-dstnodata value [value...]]
    [-rn] [-rb] [-rc] [-rcs] [-wm memory_in_mb] [-multi] [-q]
    [-of format] [-co "NAME=VALUE"]* srcfile dstfile


DESCRIPTION
  The gdalwarp utility is a simple image reprojection and warping
  utility. The program can reproject to any support projection, and can
  also apply GCPs stored with the image if the image is 'raw' with
  control information."

--
Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer
"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!"




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