DECLARE @NLCount INT = (
    SELECT COUNT([LAT_N])
    FROM STATION
    WHERE [LAT_N] IS NOT NULL
);

DECLARE @NLArray TABLE (
    [ARR_INDEX] INT IDENTITY(1,1),
    [LAT_N] DECIMAL(10,4)
);

INSERT INTO @NLArray
SELECT [LAT_N]
FROM STATION
WHERE [LAT_N] IS NOT NULL
ORDER BY [LAT_N];

SELECT CAST(
CASE
    WHEN (@NLCount % 2 = 0) THEN (
        SELECT AVG([LAT_N])
        FROM @NLArray
        WHERE [ARR_INDEX] IN (
            @NLCount / 2,
            (@NLCount / 2) + 1
        )
    )
    ELSE (
        SELECT [LAT_N]
        FROM @NLArray
        WHERE [ARR_INDEX] = (@NLCount + 1) / 2
    )
END AS DECIMAL(10,4)) AS Median;