OSGBI 1858 could be the Liverpool datum. The first O.S. selected datum point in
1840 was 100 feet below a bench mark on St. John's church, Liverpool, and a few
years later at the Victoria Docks where tidal observations were taken every five
minutes. From 1844 the approximate sea level was adopted from the Liverpool
datum and was the accepted reading for 80 years, i.e. until the Newlyn datum was
adopted from hourly tide readings 1915-1921 see
http://www.newlyn.info/content/view/127/51/

Gerry Nichols


From: noiseismusic
Sent: Tuesday, May 06, 2008 5:31 PM
To: ordnancemaps@...
Subject: Re: [ordnancemaps] Cassini-Soldner projection and Geographic Coordinate
System


Just to update people on my Cassini-Soldner projection creation issues with
ArcGIS. I now have my six inch map referenced as required. I am happy to
e-mail people details of how I set this up in ArcGIS, along with some of my
calculations. However I have explained below where I was going wrong and
that may be help others avoid such simple but important mistakes.

I still have one question though. Brian Adams in his article, Ordnance
Survey County Series plans - sheet line data (page 71. of the book
Projections
and Origins), states that the spheroid is Airy and the horizontal datum
OSGBI 1858. Does any one know what OSGBI 1858 refers to? I've not been able
to find anything about it on the Internet. I am currently using Airy 1830
for both the datum and spheroid and that appears to work.

The main problems I previously encountered were using the wrong measurement
system and forgetting that anything west or south is negative, not positive.
When I created the world coordinate file, tfw file, to position the map, I
originally used the latitude and longitude of the top left corner of the
scan. The actual coordinates required, were the distance from the projection
origin, that the corner point was located. This needed to be in feet when
the linear unit was set to feet in the projection and in metres when the
linear unit was set to metres in the projection. However the scale factor in
the tfw file, lines 1 and 4, need to be calculated using feet, regardless of
whether your linear unit is in feet or metres. Also make sure that your tfw
file is created and titled the same as your image file, before you create
the pyramids in ArcGIS, otherwise it will not work. Once I did all of that
it worked.

There was one issue I did have though. When I did the above with my linear
unit set to feet and the distance from origin calculated in feet, the map
loaded into ArcGIS with the scale box greyed out showing 0:0. When I did the
above with my linear unit set to metres and the distance from origin also
calculated in metes, the map loaded into ArcGIS with the scale box showing
the correct scale of 1:61,679, which is the scale of the map when fully
displayed on my computer screen. Why that is happening, I do not know, but I
do not have time to find out. It is actually more helpful to have the linear
scale in metres for my work.

Having said it worked, it still is not 100% perfect. Throughout I had been
comparing my work to historical Ordnance Survey maps, that were already
referenced and projected using the Transverse Macator projection. These I
was able to get through my universities Edina Digimap service. People
without access to such a service will find this work much harder to do, as
you will have nothing to compare your work with. Once I had set everything
up, it showed that my map was not in the exact same position as the
OS/Landmark group version but that it was very close. The differences may be
caused by my scale factor in the tfw file being different and the scanning
distortions that exist in my scanned map. As the OS map was projected using
the Transverse Macator projection and my map is projected using the
Cassini-Soldner projection, that may be another factor. As I am only working
from this one sheet though, these issues will not be a major problem.

Using ArcGIS, I now intend to geo-reference other maps to the map I have set
up. These are maps, which like most town plans, do not have latitude and
longitude coordinates on them. By working in this way, none of my work will
been derived from still in copyright Ordnance Survey data. As their
downloadable historical data is copyrighted, I have only been using that as
a source to compare my map to, which I believe isn't against the law.

King regards

Tim





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