[Xastir] Maps, again!

[John Ronan]

On 22 Oct 2004, at 21:15, Gerry Creager n5jxs wrote:

> Aha.  the pages referenced below simply did the leg-work for you.
>
Yup, I'
> Curt,  We need to talk about postgis again.  Some of this pain can be 
> dealt with directly therein.
>
> John,  Find the metadata associated with the images you have gotten. 
> See what the center point and the corners are supposed to be at... 
> you've pretty well gotten there.  Identify the datum associated with 
> the geodetic position, and the coordinate system.  Determine if the 
> image is appropriately rectified.  Also, in the registration process 
> the degree of affine used (number of points) should be given.  
> Finally, did the supplier project the image to match a particular 
> maptype?  This is uncommon but not unheard of.  If so, you may have to 
> reproject it, which is a pain.  It'll likely mean, more to the point, 
> doing a rubber-sheet re-registration/rectification of the image which 
> will introduce some distortion.
>
Jeez, I need to wakeup first I'll read the above again.

Basically I got a friend of mine to scan in a part of the O.S. Map for 
my area (he has a very good scanner), we then cropped it at the 
appropriate points. So assuming that the scanner didn't introduce any 
more errors, then the grid on the map is visible every 1000m so they 
are the points I could use to try and match things up.

Re the map data itself. From OS themselves
http://www.osi.ie/gps/overview/irishgrid.asp

Also I had a though re the 'stitching' of the maps while on my return 
journey last night.  Its really not necessary for me to do it (but I'd 
still like to try).  As the event I'm trying to get organised for is an 
event where we give support to the Hillwalking association. We are 
planning on putting 2 trackers per walk, one per transport vehicle and 
one one the Mountain Rescue Vehicle.  Basically the entire mountain 
range will be on one scanned section, so all the 'action' will be in a 
relatively small area of the map. So I don't really need to do any 
stitching.


Curt I'll look at the pocketAPRS maps as well.

Thanks for the replies.

I'll go get breakfast and come back and read them again when I'm fully 
awake.

Regards
John




> 73, gerry
>
> John Ronan wrote:
>> Hi There again,
>> I'm about to head up to my folks place (30 miles or so - EI7IG-9), 
>> I've been busy putting in the sections of the map so that I can see 
>> it afterwards.
>> Each Tiff is 4724 x 4724 and the boundarys are the lines in the map.  
>> So I've been using
>> http://www.megalithomania.com/conversion/
>> To convert from Irish Grid to Degrees Minutes Seconds and then
>> http://www.uky.edu/KGS/gis/converter.htm
>> to convert to decimal Lat/Long.
>> I'm wondering what others use to convert to decimal lat/long as I'm 
>> seeing gaps/slight overlaps in the pictures.
>> Tomorrow I'll check it out a bit more, but It looks to me like a 
>> rounding error of some sort as two of the maps (I've done a square) 
>> that I have done have the northeast/southwest corners at the correct 
>> point.
>> Bearing in mind that I know little or nothing about maps except that 
>> they are all a 'projection' of some sort.  Can I assume that as I go 
>> further and further from the 'point' mentioned above that I will 
>> begin to see gaps in the projection.
>> Thanks for the help (and a great application)
> -- 
> Gerry Creager -- gerry.creager at tamu.edu
> Network Engineering -- AATLT, Texas A&M University	
> Cell: 979.229.5301 Office: 979.458.4020
> FAX:  979.847.8578 Pager:  979.228.0173
> Office: 903A Eller Bldg, TAMU, College Station, TX 77843
>
--
John Ronan <jronan at tssg.org>, +353-51-302938
Telecommunications Software &  Systems Group,  http://www.tssg.org