Difference between intensity value and digital number in remote sensing

Question

I have very basic question regarding remote sensing.

What is the difference between intesity value of a pixel and digital number of the pixel in case of remote sensing. Are these two terms same or different?

If they are different, how to do we transform from one to the other?

Answer 1

Intensity value refers to the actual physical measurement of the energy radiated from a surface. It is usually measured in units such as watts per square meter (W/m2).

On the other hand, digital number (DN) is a numerical representation of the intensity value, which is obtained by converting the analog signal from the sensor to a digital signal. DN values are usually integers and are used to represent the intensity of the energy radiated or reflected by the surface or object. DN values can range from 0 to a maximum value, which depends on the bit depth of the sensor. For example, a sensor with a bit depth of 8 can produce DN values ranging from 0 to 255.

Answer 2

Raw digital numbers are the values recorded by the sensor before any conversion to more informative units such as reflectance. Intensity is a way to gauge the relative strength of the signal that is received by the sensor compared to other signals in the scene. Conversion to reflectance is desirable because this is a property of the material being sensed rather than a property of the signal itself like intensity. Intensity values are recorded as digital numbers, but reflectance values exist on a percentage scale.

Intensity values are constrained by the bit depth of the sensor array, which governs how many possible digital number values exist between no signal received and maximum intensity (saturated signal). There are some differences in interpretation based on the type of sensor being used. For example, in the case of Lidar data, a signal corresponds to an individual laser return, and in the case of images the signal corresponds to an individual pixel.

Answer 3

It depends on your data. Intensity is a term more commonly used in laser scanning. Since you referred to pixels, I assume you want to know how it goes for image data.

The digital number (DN) is the raw output of the sensor that needs to be converted to a physical quantity (e.g. radiance or reflectance) to allow further processing. For this you need to know some things about your sensor. This can be done either by using a sensor that is well known, e.g. from NASA's Landsat mission, or ESA's Sentinel mission, or by calibrating your own sensor/camera (cannot give guidance on this). For the example of Landsat, the conversion from DN to radiance/reflectance is a simple linear model reflectance = a + b * DN, with a and b known. See section 5 of https://landsat.usgs.gov/sites/default/files/documents/Landsat8DataUsersHandbook.pdf for more.

Also see the answer https://gis.stackexchange.com/a/173631/134898 for more details, and https://www.youtube.com/watch?v=sOk5fFPSDBA for a better explanation of DN, radiance etc.

Answer 4

It depends. Digital number only tells you that it is a value without a unit that usually is acquired through digitization and it is usually limited to a range like 0-255.

So in your case it should be the same.