NBR difference in Google Earth Engine

Question

I want to use two NBR images(NBR1 and NBR2) for the differenced Normalized Burn Ratio (dNBR). But I can't identify the error of dNBR output.

Code snippet:

var NBR1 = landsat8_1.expression(
        "(NIR - SWIR2) / (NIR + SWIR2)",
        {
          SWIR2: landsat8_1.select("B7"),    //SWIR2
          NIR: landsat8_1.select("B5"),    // NIR
          BLUE: landsat8_1.select("B2")    // BLUE
        });

var NBR2 = landsat8_2.expression(
        "(NIR - SWIR2) / (NIR + SWIR2)",
        {
          SWIR2: landsat8_1.select("B7"),    //SWIR2
          NIR: landsat8_1.select("B5"),    // NIR
          BLUE: landsat8_1.select("B2")    // BLUE
        });

var dNBR = NBR1.subtract(NBR2);

Full code: code.earthengine.google.com/14aaf46d74850fec158b3485c6fe702c

Answer 1

Well, always check your code and debug it. Look your code:

var NBR1 = landsat8_1.expression(
        "(NIR - SWIR2) / (NIR + SWIR2)",
        {
          SWIR2: landsat8_1.select("B7"),    //SWIR2
          NIR: landsat8_1.select("B5"),    // NIR
          BLUE: landsat8_1.select("B2")    // BLUE
        });

var NBR2 = landsat8_2.expression(
        "(NIR - SWIR2) / (NIR + SWIR2)",
        {
          SWIR2: landsat8_1.select("B7"),    //SWIR2
          NIR: landsat8_1.select("B5"),    // NIR
          BLUE: landsat8_1.select("B2")    // BLUE
        });

You are applying the same expression with the same image and storing it in two different objects. That's why the output of dNBR is empty. Modify it and you will have the desired output:

var NBR1 = landsat8_1.expression(
        "(NIR - SWIR2) / (NIR + SWIR2)",
        {
          SWIR2: landsat8_1.select("B7"),    //SWIR2
          NIR: landsat8_1.select("B5"),    // NIR
          BLUE: landsat8_1.select("B2")    // BLUE
        });

var NBR2 = landsat8_2.expression(
        "(NIR - SWIR2) / (NIR + SWIR2)",
        {
          SWIR2: landsat8_2.select("B7"),    //SWIR2 of 2nd image
          NIR: landsat8_2.select("B5"),    // NIR of 2nd image
          BLUE: landsat8_2.select("B2")    // BLUE of 2nd image
        });

var dNBR = NBR1.subtract(NBR2);

Please, always check your code twice. This is a common mistake