Coordinate conversion

#1 SII Posted 28 July 2008 - 12:29 PM

SII

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• Location:Ghana
• Ghana

Please can anyone help me derive an equation or relation that can help me convert new GPS points in decimal degrees to XY coordinates.

Please i am a biomedical science student from Ghana working on developing Malaria risk map for my study area, in a study involving studies on malaria transmission. I am new in GIS and I need help with my project work. I want to derive an equation or relation which I can use to convert my coordinates in decimal degrees to XY coordinates. Below is an example of a work that had been done earlier that converted the degrees to XY.

ID X Y ID Long (N) Lat.(W)
A1 221915.447 211557.864 A1 1.5675 6.6764
A2 221976.838 211588.643 A2 1.5672 6.677
A3 222007.551 211588.678 A3 1.5671 6.6773
A4 222774.603 212265.118 A4 1.5612 6.6841
A5 222897.42 212295.965 A5 1.5609 6.6852
A6 221331.864 211587.9 A6 1.5672 6.6711
A7 222592.383 210483.89 A7 1.5771 6.6825
A8 221086.265 211495.494 A8 1.5681 6.669
A9 220994.41 211249.723 A9 1.5703 6.668
A10 221086.584 211219.121 A10 1.5705 6.6689
A11 221362.577 211587.936 A11 1.5671 6.6715
A12 221055.871 211219.086 A12 1.5706 6.6685
A13 221239.584 211710.626 A13 1.566 6.6704
A14 221300.869 211833.528 A14 1.5651 6.6707
A15 221611.015 209254.425 A15 1.5884 6.6736
A16 221393.114 211741.51 A16 1.5657 6.6717
A17 222286.485 209439.477 A17 1.5868 6.6796
A18 221856.647 209316.134 A18 1.5878 6.6759
A19 221025.052 211311.175 A19 1.5696 6.6684
A20 222928.273 212173.172 A20 1.5619 6.6856

I learnt that the systems that was used is called the War Office. Please i am not so familiar with these tools but i need it so i can convert new points. Below is an example of new points that i have to convert

ID Long(N) Lat.(W)
A1 1.5703 6.668
A2 1.5705 6.6689
A3 1.5671 6.6715
A5 1.5706 6.6685

I would be grateful if i can be helped.
Thank you

#2 Hans van der Maarel Posted 28 July 2008 - 12:54 PM

Hans van der Maarel

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Using FME, with the Accra.GhanaNational projection, I'm getting results pretty close to your reference data:

ID,Long,Lat,X,Y

A1,-1.5703,6.66800,220942,211248

A2,-1.5705,6.66890,221041,211226

A3,-1.5671,6.67150,221328,211602

A5,-1.5706,6.66850,220997,211215

For reference, I also transformed the A20 record from your reference set, which came out as 222887,212179 instead of 222928,212173. It would be logical to assume the other values have similar accuracies (and 4 significant digits in your GPS coordinates would indeed mean an accuracy of around 100 meters)

Hope this helps!
Hans van der Maarel - Cartotalk Editor
Red Geographics