Friday, January 20, 2012

1:50000 map index

L7018 Index around Ko Samui
It was an innocent question by Tom Isaan on whether I know a simple way of how to find the right 1:50000 map sheet when from the geographical coordinates which should be shown. The Royal Thai Survey Department (กรมแผนที่ทหาร) has its 1:50000 topographic maps indexed with a four digit number and a Roman numeral from I to IV, and at first look the index map looks relatively straightforward - every map covers an area of 15 arc minutes in longitude as well as in latitude, and start at either the exact degree line, or 15, 30 or 45 arc minutes offset. And the number and roman numeral follow a logical order as well - the first two digits encode the longitude in half degree steps, the next two the latitude, and the Roman numeral finally the four sheets within the half degree square. If it were just this, it'd be easy, only would have to make sure that the algorithm does not return squares outside the country area and thus not covered by any map in this series.

But starting at around Hua Hin it gets messy. Due to the narrow but tilted Isthmus some of the sheets have a different longitude offset, in order to reduce the number of sheets necessary. And further down one even finds sheets more wide than the 15 arc minutes, and for Ko Samui and Ko Pha Nga the maps cover more than 15" in latitude - see the detail of the index map above. And finally, the map with the Tarutao archipelago has both latitude and longitude at an odd offset.

Since there are only the PDF index files, but no list with the actual coordinates for each sheet, I had to measure in the PDF to get the special sized or located sheets, thus I am not completely sure if I used the correct offsets in all cases. But at least it comes close, the result is seen in the map below.

View Larger Map or download KML

What is left to do? Obviously, I need to compare my coordinates of those sheets with unusual offsets with those actually printed on the maps, and also check whether L7018 and L7017 are completely the same (except that L7018 uses WGS84 instead of Indian75 as the map datum) - from the index map it looks like the Tarutao sheet 4822 II was changed. Other things I could add are the Bangkok 1:20000 maps, and the 1:250000 1501 series. And also interesting would be to see what was done at those parts which seem to be not covered by any of the L7018 maps, for example the area around Tha Song Yang district office - it might be in placed in the Burmese area of 4643 II, but without seeing that map I can only say that it seems there is no 1:50,000 map of that area.

I will publish a new version of my coordinate conversion tool to include the map indices once I could address those open issues.

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