Read bmp

HELLO,
i need to Extract the thermal height from the map "XC.bmp" (Maximum map size 800x800 pixels can be assumed - the map size will change and read from the file).
and create a "therm_res.txt" coordinate file with their location and updated heights .
each pixel on the map represents 100 meters**

Blue	131	93	51	125	255	255	189	189	113
Green	255	215	138	93	134	213	153	98	70
Red	177	93	53	51	52	129	165	199	119
Height	2700	2400	2100	1800	1500	1200	900	600	300

How should I move from here?

Code:

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#include <stdio.h>
#include <math.h>
#include<Windows.h>


int read_bmp() {
        unsigned int size = 0;
        int i;
        FILE * picXC;
        fopen_s(&picXC, "XC.bmp", "rb");
        if (!picXC) {
                printf("Problem reading XC.bmp file\problems with setting heights");
        }
        else for (i = 0; i<4; i++) size |= fgetc(picXC) << (i * 8);
        // read into "therm_res.txt"
        fclose(picXC);
        return 0;
}

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I guess you read this, and figure out what all the bytes mean.
BMP file format - Wikipedia

Start by trying to print the width and height of a range of bmp files.