Hello Ibrahim, this appears to be a really tricky and interesting inquiry. Just from my lay understanding, I would speculate that a number of considerations come to play. Among these I would opt to interrogate the following:

1. The latitude of the subject area - this is based on the angle of inclination of short wave radiation striking the surface which varies laterally among different regions of the world (based on proximity or lack thereof to the equator) and the season of the year in which the study is being conducted;

2. The altitude of the study area - coupled with the time of the day, this would help to sieve out intensity differentials to enable you arrive at an informed average;

3. Land cover - surface characteristics do exert a significant bearing on energy receipts and determines the area's reflectivity index / (albedo) that helps to appreciate the terrestrial radiation losses. Built up areas with intensity of human activities boast of high emissivity, depending on the dominant activities. For areas near large water bodies, the moderating effect of the waters could also be taken note of;

4. Air masses, for near surface motions, air masses could manage to interfere with an area's energy concentration as lateral dispersion attempts to ' create a compromise' between energy surplus and deficit regions.

In view of the above, I would suggest a load of possible distortions would need to be taken care of to allow for a relative quantification of the land surface emissivity. The issue would be how to account for the contribution of each of these factors to the observed emissivity.