In Nepal we use 3 degrees width zone UTM with 3 central meridians 81 E, 84 E and 87 E and datum: Everest-1830. I am trying to convert the Everest-projected coordinates to WGS-84's Latitude and Longitude. I applied reverse of projection for conversion of Everest projected Easting and Northing to Everest's Latitude and Longitude and then transformed to WGS-84's Latitude and Longitude
I used the formula in ogp publications 373-7-2 geomatics guidance note number 7 page number 50 and 54 to calculate latitude and longitude from Easting and Northing Projected coordinates and created code to do so.
But whenever I tested the conversion and input Easting, Northing and height values 617000, 3038250, 1230 respectively of zone 88 (Central meridian 84 degree E, datum : Everest-1830). I get output latitude 27.461834699447184 and longitude 83.86573999170062 (Datum: Wgs-84). But the expected output is latitude: 27.732 and longitude: 85.417.
I must be doing something wrong. Please help me