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I'm not a GIS professional.

I'm trying to write a code to convert from lat/long to UTM coordinates using this truncated method given in Wikipedia:

https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system#Simplified_formulae

What values I should use for lambda_0 (reference meridian of longitude)?

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    UTM is divided into zones, with each zone (presumably) having a different reference meridian of longitude. You'll need to determine the UTM zone projection of your coordinates to find the lambda_0, I believe. – Jon Apr 12 '18 at 19:05
  • Welcome to GIS SE! As a new user be sure to take the Tour. For questions that involve code we ask that you show us where you are stuck with your own code by including a code snippet in your question. There is an edit button beneath your question which will enable you to do that and a {} button that enables you to format any highlighted code nicely. – PolyGeo Apr 12 '18 at 21:30
  • Question and answer on find the zone based on a lat,lon point: gis.stackexchange.com/questions/13291/… – mkennedy Apr 12 '18 at 21:45
  • please, use one of the many well known and supported libraries that already exist to do this. See proj4.org for example – Ian Turton Apr 13 '18 at 9:10
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You can calculate the reference meridian and the UTM zone from the longitude of your point like this :

Zone = Floor((long + 186°) / 6))

lambda_0 = Zone * 6° - 183°

Make sure to keep the Zone and Hemisphere information along with each pair of E-N coordinates, especially if your points span several zones.

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