I have downloaded MODIS 8 day average LST images, but the values are way too high. How do I convert the pixel values to degree celsius? Someone said they multiplied the data with 0,02 and then subtracted 273 to get them as Celsius. is that the correct method?
Information on how to convert the digital values ("digital numbers" or DN) in satellite remote sensing data that is operationally produced and distributed into physical quantities of interest is generally found in the data user guide. In this case, you need the user guide for the MODIS 11 Level 2 product. Under "scientific data sets (SDS" there is a table that contains the relevant information for the land surface temperature:
SDS Name: LST Long Name: Land-surface temperature Number Type: uint16 Unit: K Valid Range: 7500-65535 Fill Value: 0 scale factor: 0.02 add offset: 0.0
"Scale factor" and "add offset" are used for a linear re-scaling of the DN values (which are going to be between 7500 and 65535) to temperatures in K. As the page explains:
The effective calibration formula for the "LST" SDS is LST = the SDS data in uint16 * 0.02, giving a value in the range of 150-1310.7K.
So the person who gave you advice was almost right: Temperature in °C will be:
DN * 0.02 - 273.15.
It must be remembered that the null values must be taken into account, which must be changed to zero and the method of interpolation used.
You can see it here: https://stevemosher.wordpress.com/modis-reprojection-tool/