Some weeks ago, I found
this great Tableau formula by Rik van Schaik to convert x,y co-ordinates from the Dutch Rijksdriehoek system to latitude, longitude.
Works great in PostgreSQL to:
-- Based on a Tableau calculated field formula by Rik Van Schaik
-- Added more decimals for precision
-- Rewritten for PostgresSQL
-- Replace x & y by field names of your choice
-- feb-2020 aukema.org
select
52.15517440 + ((
(3235.65389 * ((y - 463000) * 10 ^ -5)) +
(-32.58297 * ((x - 155000) * 10 ^ -5) ^ 2) +
(-0.2475 * ((y - 463000) * 10 ^ -5) ^ 2) +
(-0.84978 * ((x - 155000) * 10 ^ -5) ^ 2 * ((y - 463000) * 10 ^ -5)) +
(-0.0655 * ((y - 463000) * 10 ^ -5) ^ 3) +
(-0.01709 * ((x - 155000) * 10 ^ -5) ^ 2 * ((y - 463000) * 10 ^ -5) ^ 2) +
(-0.00738 * ((x - 155000) * 10 ^ -5)) +
(0.0053 * ((x - 155000) * 10 ^ -5) ^ 4) +
(-0.00039 * ((x - 155000) * 10 ^ -5) ^ 2 * ((y - 463000) * 10 ^ -5) ^ 3) +
(0.00033 * ((x - 155000) * 10 ^ -5) ^ 4 * ((y - 463000) * 10 ^ -5)) +
(-0.00012 * ((x - 155000) * 10 ^ -5) * ((y - 463000) * 10 ^ -5))
) / 3600) as lat,
5.38720621 + ((
(5260.52916 * ((x - 155000) * 10 ^ -5)) +
(105.94684 * ((x - 155000) * 10 ^ -5) * ((y - 463000) * 10 ^ -5)) +
(2.45656 * ((x - 155000) * 10 ^ -5) * ((y - 463000) * 10 ^ -5) ^ 2) +
(-0.81885 * ((x - 155000) * 10 ^ -5) ^ 3) +
(0.05594 * ((x - 155000) * 10 ^ -5) * ((y - 463000) * 10 ^ -5) ^ 3) +
(-0.05607 * ((x - 155000) * 10 ^ -5) ^ 3 * ((y - 463000) * 10 ^ -5)) +
(0.01199 * ((y - 463000) * 10 ^ -5)) +
(-0.00256 * ((x - 155000) * 10 ^ -5) ^ 3 * ((y - 463000) * 10 ^ -5) ^ 2) +
(0.00128 * ((x - 155000) * 10 ^ -5) * ((y - 463000) * 10 ^ -5) ^ 4) +
(0.00022 * ((y - 463000) * 10 ^ -5) ^ 2) +
(-0.00022 * ((x - 155000) * 10 ^ -5) ^ 2) +
(0.00026 * ((x - 155000) * 10 ^ -5) ^ 5)
) / 3600) as lon