PATTERNS

scalar operations

Uniformly change the values of all data in a raster object by adding, subtracting, multiplying, or dividing the raster by a constant.

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add

Add each pixel value by constant. For example, pattern below adds by one.

var image_add_constant = image.add(1)

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subtract

Subtract each pixel value by constant. For example, pattern below subtracts by 1.

var image_subtract_constant = image.subtract(1)

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multiply

Multiply each pixel value by constant. For example, pattern below multiplies by 2.

var image_multiply_constant = image.multiply(2)

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divide

Divide each pixel value by constant. For example, pattern below divides by 2.

var image_divide_constant = image.divide(2)

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problem types

Here are a few types of problems that can be solved with scalar operations.

change value units

A common example is when you need to change the units of your data. For example, to change elevation data from centimeters to meters you divide all elevation values by the number 100 (scalar).

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graph LR
  input["image_cm"] ;
  method(".divide()") ;
  output[/"image_m"/]  ;
  arg1["100"]  ;


  input --> method  
  method --> output
  arg1 --o method

  classDef in-out fill:#FFFFFF,stroke-width:1px,stroke: #000000, color:#000000; 
  classDef op fill:#000000,stroke-width:0px,color:#FFFFFF;
  classDef arg fill:#CCCCCC,stroke-width:0px,color:#000000;

  class input in-out;
  class method op;
  class output in-out;
  class arg1 arg;

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var image_m = image_cm.divide(100);

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The table below lists some common types of unit conversions and their pattern context.

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CONTEXT	INPUT UNITS	OUTPUT UNITS	METHOD
TERRAIN	cm	m	.divide(100)
TERRAIN	ft	m	.multiply(3.28084)
SLOPE	degrees	percent	.divide(180).multiply(Math.PI).tan().multiply(100)

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vertical exaggeration

Another common example is when you want to apply vertical exaggeration to a terrain operation by multiplying the elevation values by a constant, usually called the z-factor. For example, by multiplying elevation by 2, you will exaggerate the terrain, making every location appear twice as high as it ‘really’ is. It is often helpful to exaggerate terrain when visualizing micro-topography at large scales or macro-topography at small scales.

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graph LR
  input["image_m"] ;
  method(".multiply()") ;
  output[/"image_m_ve_2"/]  ;
  arg1["2"]  ;


  input --> method  
  method --> output
  arg1 --o method

  classDef in-out fill:#FFFFFF,stroke-width:1px,stroke: #000000, color:#000000; 
  classDef op fill:#000000,stroke-width:0px,color:#FFFFFF;
  classDef arg fill:#CCCCCC,stroke-width:0px,color:#000000;

  class input in-out;
  class method op;
  class output in-out;
  class arg1 arg;

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var image_m_ve_2 = image_m.multiply(2);

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