Converting between Tasmap 25k Map numbers and coordinates

[Son of a Beach]

I have a need to write a small script to do two things:
• Input a coordinate, and output the Tasmap 25k map number on which that would be found
• Input a Tasmap 25k map number and output the coordinates that the map covers

Ie, translate both directions between map number and coordinates.

For now, I'll ignore all of the map numbers with a odd number as their second digit. These appear to cover all of the maps with odd sizes or shapes or which otherwise don't fit into the standard grid. It appears that all map numbers with an even number second digit are part of the standard grid, according to my quick look at Tasmaps index of 25k maps. So I'll just stick to the standard grid maps, and will have to treat the others as special cases, outside of the standard calculation.

I don't have any of my maps handy right now, but I'd like to know if there's a reliable and accurate means of doing this coordinate/map-number translation by a standard calculation. Ie, does each of these maps cover exactly the same range of coordinates both horizontally and vertically? If so, what is the coordinate of the top left of the entire grid (top left of map 2260, "New Year"), and the bottom right of the entire grid (bottom right of the non-existent map that would theoretically cover the area of sea with map number 6217).

Does this make sense?

Does anyone have this information that they'd care to share with me?

[photohiker]

Hi.

I have no idea about how to do that, but I have noticed while looking at OS Scotland mapping that they suffer the same problems with map numbers. I guess it would require a lookup table of co-ordinates for the corners of each map and a bunch of fancy maths

In a related note that may or may not help, I recently found Where's the Path? which is an online route planning site for Scotland (possibly the whole UK) which has the OS 25k maps alongside and synchronised with Google (and other) Maps. It's an awesome resource, would be great to have something like that for Tas. If you look at the bottom of the Google side, you will notice the cursor co-ords are shown in Map Co-ords as well as WGS84 so it looks like someone has had to do this sort of thing before.

[Son of a Beach]

I've written to Tasmap (DPIWE) asking if they have a table or any such reference for determining which maps cover which range of coordinates. I'm not holding my breath, but they ought to have something somewhere.

[Byrnout]

Hi SOB,

First post here, so go easy on me!

Not an expert, but have been doing a bit of mapping lately for work, so here goes on my rough thoughts!

I believe that the majority of the maps (as you can see by the index page on tasmap) are in the same format and cover 10km by 20km. The map number refers to the corresponding easting and northing for the position on the bottom left of the map - i.e Arthurs Lake 4835 is 48000mE 5350000mN, and given that is covers 10,000m north and 20,000m east from that point, then the cooridinates for top left, top right and bottom right would be - TL (48000mE, 5360000mN), TR (50000mE, 5360000mN), BR (50000mE, 5350000mN) - if that makes any sense?!

This is based on the assumption that the coordinates you are working in is UTM. It should not be too hard to write a program based on this?

Don't know if that helps at all, I have only just got it all sorted in my head recently!

Cheers,

Nik.

[Son of a Beach]

Hi Byrnout.... welcome, and thanks for the useful input... good first post!

Before reading your post, I'd just finally looked at some of my maps at home, and realised that they are Universal Transverse Mercator projection. This means that the sides of the map are NOT parallel to the lines of longitude, which is my first setback. I need coordinates in latitude and longitude, and if the sides of the map don't fit neatly into this coordinate system, then that throws out the accuracy a bit. But only a very little bit, and the difference per map is tiny - it's the calculation on the whole Tasmanian grid that is going to be significantly out.

However, your post has thrown light on the numbering system, and that is way cool, and is clearly going to be good for calculations.

So it looks like I'm going to have to base all my calculations on UTM grid references, and convert them to latitude and longitude. I'm sure there must be some standard calculation for such conversion. I'll have to do some more research on that.

[Son of a Beach]

This looks like it could be useful. I'll have to read through all that properly later, but it looks like it includes conversion information (and tools) for Tasmania.

You GPS people probably know all this but it's all new to me. Including the King Island problem explained on the site linked above. I'm starting to understand it all now, and I understand at least some of the reason for the various different grid formats at last.

[north-north-west]

... and I understand at least some of the reason for the various different grid formats at last.

I'd always assumed it was simply to create the maximum amount of confusion possible.

[BarryJ]

I don't know whether it is of any use but I use a free little app (details below) to convert UTM co-ords into Long/Lat & vice versa.

[Son of a Beach]

What I really want is to be able to write scripts that do this for me. I want to be able to enter a map number (or submit a map file), and have the latitude/longitude coordinates range output (or have the file renamed to include the latitude/longitude in the file name).

Actually, what I really want is to have the image file automatically split up into tiles of less than 1000 pixels, and each one of these smaller files appropriately named with their latitude/longitude coordinate ranges.

Now that I have all the understanding of how these coordinate systems work, I now know what to search for. I've found that there's a PERL module for UTM lat/long conversions on the standard PERL module repository. This is exactly what I need.

I've already written a GIMP script that splits the large image file into tiles as required, and now I'll be able to write a PERL script that renames each tile with latitude/longitude ranges for their file names.

I think I now have all the pieces I need to put the puzzle together now. I've just got to find the time to do it.

Thanks guys for your help. It's been an education for me!

[Son of a Beach]

I think I've done it! I've written a PERL script which will take a file with a name in the format:

Code:

Ellipsoid/Datum Zone Easting1 Northing1 Easting2 Northing2.ext

such as:

Code:

Australian_National 55G 50000 5410000 52000 5420000.jpg

(ie, the "Dilston" 5041 map) and rename it to:

Code:

-41.3356274595542 141.622984663982 -41.2470557118148 141.654130181725.jpg

I'll need to confirm if those are in fact the correct lat/long coordinates for the Dilston map when I get back home (although -41... certainly sounds about right).

Combined with my existing GIMP script to split an image into tiles, and a few more additions to manipulate file names of those tiles, I'm all set!

PS. I've subsequently written a second script which will take a file named after the Tasmap map number, (eg, "5041.jpg") and rename it to a file named after the latitude and longitude coordinates.