The Raster Calculator allows the user to perform Raster Calculations on remotely sensed imagery To initiate a Raster Calculation, go to the Analysis menu and select Raster Calculator - Apply Formula (NDVI, NDWI, etc) to Image/Grid Layer(s).
If more than one raster layer is loaded, the Select Layers window will appear.
Check the raster layers to select the data for analysis.
Click ok and the Set up Apply Formula Operation window
will appear.
The layer(s) selected for Raster Calculation will show in Layer Description.
Under Formula, the user
may choose to Add Predefined Formula, or Add
Custom Formula.
The Add Predefined Formula allows
the user to choose between NDVI and NDWI Analysis.
NDVI = (NIR-RED)/(NIR+RED)
The Normalized Difference Vegetation Index (NDVI) is a commonly used vegetation indices derived from the Near-Infrared (NIR) and RED channels of remotely sensed imagery. NDVI is an index of vegetation’s photosynthetic activity, or ‘greenness’. Vegetation indices are based on the observation that different surfaces reflect different types of light differently.
The NDVI is calculated by determining the ratio of red and near infrared bands from a remotely-sensed image on a per-pixel basis to use as the normalized difference between red and near infrared bands in an image.
There are a couple of predefined options for NDVI, depending on which band NIR and RED are found in.
NDVI (RGBI): NIR = B4, RED=B1
NDVI (IBGR): NIR=B1, RED= B4NDWI = (G-NIR)/(G+NIR)
The Normalized Difference Water Index (NDWI) is derived from the Near-Infrared (NIR) and Green (G) channels. This formula highlights the amount of water in water bodies.
An alternate method of calculation uses the NIR and Short Wave Infrared (SWIR) channels [(NIR-SWIR)/(NIR+SWIR)]. The amount of water present in vegetation primarily affects the spectral reflectance in the SWIR channel. The information about vegetation contained in the SWIR channel is unique. NDWI should be considered as an independent vegetation index.
NDWI : G=B2, NIR=B4
NDWI (IBGR): G= B3, NIR=B1
NDWI (RGBI): G=B2, NIR=B4NBR = (NIR-SWIR)/(NIR+SWIR)
The Normalized Burn Ratio calculation is used to highlight burned areas and to estimate severity of fire. Burned areas reflect high in shortwave infrared (SWIR) and low in near infrared (NIR) and will therefore have a low NBR value. Similar to NDVI, healthy vegetation will have a high NBR value. The band numbers (B4, B6) are based on Landsat 4-5 band designations, but the formula may be used for any imagery by specifying the NIR band as B4 and the SWIR band as B6 in the subsequent dialog.
NBR [Normalized Burn Ratio]: NIR=B4, SWIR/Thermal=B6
The Add Custom Formula allows the user to input the raster or 'band math' for custom calculations. Bands should be specified as B1 for Band 1, B2 for Band 2, and so forth. See Custom Formula Reference
When the Add Band button is clicked, the band selection dialog will ask which specific bands from the loaded layers correspond to the bands in the formula(s).
Set
up Output Grid Settings allows the user
to specify the bit depth of a shaded single band grid of the new band,
or to create a Multi-Band image as output. The Shader option
can be used to specify that a predefined or custom Shader should be applied
to the image, rather than the default NDVI shader.
Use 'Z' in the equations for the elevation when providing a formula for
terrain layers (B1 will also work). The selected input layers must be
either all elevation or all raster imagery and the appropriate mode (i.e.
from grids or from imagery).
If all inputs are grids then the output can only be a single band/ formula
targeting a grid format.
Custom Formulas allow you to generate new bands for a raster image based on formulas that are not already predefined in Global Mapper. Formulas are mathematical expressions that can combine numbers, band values, or elevations to compute new band values. They may use standard mathematical operators like +, -, *, /, ^ (power), comparison and relational operators, and functions like LOG, LOG10, MIN, and MAX to produce new values.
OperandsBand values - Band values refer to a specific raster band in an image. Allowable band references (again, the names below are case-insensitive):
Formulas use various mathematical and logical operators to form a result, similar to spreadsheet formulas. They are (in order of precedence, low-to-high):
You may also use parentheses to specify order of operations. In the absence of
parentheses, higher precedence operations are performed before lower precedence operations. That is, in the
formula "B1 + r * 3", the result is the value of 'B1' plus the product of 'r' and '3' (equivalently "B1 + (r * 3)").
The calculator provides several built-in functions to aid in calculation of values. Note that function names are case-insensitive; that is, log is the same as LOG