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Landsat Wipe Map

Introduction

This tutorial introduces a general workflow to make a wipe map for exploring land cover changes with satellite imagery. We will use surface reflectance data from the Landsat 5 image collection to identify spring-time scenes with few clouds of Shanghai, China that are taken 25 years apart. We will then develop natural and false color composites to display the imagery and make a false color composite that aims to be more accessible to color blind map readers. Finally, we will display the two images with a wipe map to help people compare differences between the two time steps. Our final result should look and work like the map in the app below.

Start a new script

//  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//  TITLE:      wk06_tutorial_rgb_composites.js
//  AUTHOR:     your name
//  DATE:       today's date
//  PURPOSE:    A template for comparing color composites at two times.     
//  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Start to compose a map

// ---------------------------------------------------------------------
//  Start a map.
// ---------------------------------------------------------------------

//  Define a point of interest (poi) on Shanghai.

var geometry = ee.Geometry.Point([121.46337, 31.221248]);

//  Center map on geometry (poi) and zoom level 11.



//  Set base map to satellite with labels (hybrid).

Load L5 collection

// ---------------------------------------------------------------------
//  Load Landsat 5 image collection.
// ---------------------------------------------------------------------

// Load L5 collection from this address: "LANDSAT/LT05/C02/T1_L2"

var ic ;

Work out processing routine

// ---------------------------------------------------------------------
//  Work out how to process the image collection.  
// ---------------------------------------------------------------------

// Go to the L5 page on EE Catalog and grab their script for scale factors.

// Develop the processing workflow.  

var practice ;

// Check your results. 

print(
  "Practice"
  )
;

Write routine as a function

// ---------------------------------------------------------------------
//  Wrap up the workflow as a function.  
// ---------------------------------------------------------------------

var makeImage ;

// Apply function to make base image (April 1984).

var yr1984 ;

CHECK

// ---------------------------------------------------------------------
// CHECK
// ---------------------------------------------------------------------

var check = require('users/jhowarth/eePrimer:modules/checks.js');

// print(
//   "CHECK 01:",
//   yr1984.size(),
//   "CHECK 02:",
//   check.cu06(yr1984)
//   )
// ;

Stretch Enhancement and Natural Color

// ---------------------------------------------------------------------
//  Figure out stretch enhancement.  
// ---------------------------------------------------------------------

// Import image tools module.   

var tools = require('users/jhowarth/eePrimer:modules/image_tools.js');

// Make a histogram to see data distribution.  

// var histogram = tools.makeHistogram(
//   yr1984.first(),               // Must be an image (not an image collection)
//   "SR_B3",                      // Select one band at a time.
//   30,                           // Pixel resolution of image.
//   0,                            // Minimum value of x-axis
//   0.5                           // Maximum value of x-axis.
//   )
// ;

// Print, print, print...

// print(
//   "Year 3", 
//   yr1984,
//   yr1984.first().bandNames(),
//   histogram
//   )
// ;

// Viz for natural color composite. 

var nat_viz 
;

CHECK

// ---------------------------------------------------------------------
// CHECK
// ---------------------------------------------------------------------


print(
  "CHECK 03:",
  nat_viz.min[1]
  )
;

False color

// ---------------------------------------------------------------------
// Make viz and display false color (on their own. 
// ---------------------------------------------------------------------

// Use histograms to figure out viz parameters with good contrast for 
//  "SR_B7", "SR_B4", "SR_B2" in a 742 composite.

var false_viz ;

// Display base year in false color.

Try to accommodate color blindness

// ---------------------------------------------------------------------
// Try to accommodate color blindness. 
// ---------------------------------------------------------------------

// Swap the G and B bands to avoid magenta and green.

var cb_viz ;

// Add as map layer.

CHECK

// ---------------------------------------------------------------------
// CHECK
// ---------------------------------------------------------------------

print(
  "CHECK 04:",
  false_viz.max[1],
  "CHECK 05:",
  cb_viz.max[1]
  )
;

Explore Potential Time 2 Layers

// ---------------------------------------------------------------------
//  Explore potential Time 2 layers  
// ---------------------------------------------------------------------

// How does 1994 look and work?

var yr1994 ;

// Or 2004?

var yr2004 ;

// Or 2009?

var yr2009 ;

CHECK

// ---------------------------------------------------------------------
// CHECK
// ---------------------------------------------------------------------

// print(
//   "CHECK 06:",
//   yr1994.size(),
//   "CHECK 07:",
//   check.cu06(yr1994),
//   "CHECK 08:",
//   yr2004.size(),
//   "CHECK 09:",
//   check.cu06(yr2004),
//   "CHECK 10:",
//   yr2009.size(),
//   "CHECK 11:",
//   check.cu06(yr2009)
//   )
// ;

Wipe template

// -------------------------------------------------------------------------------
// Wipe Template  
// -------------------------------------------------------------------------------

// Make another map and add a color-NIR composite to it.

var linkedMap = ui.Map();

// Link the default Map to the other map.

var linker = ui.Map.Linker([ui.root.widgets().get(0), linkedMap]);

// Create a SplitPanel which holds the linked maps side-by-side.

var splitPanel = ui.SplitPanel({
  firstPanel: linker.get(0),
  secondPanel: linker.get(1),
  orientation: 'horizontal',
  wipe: true,
  style: {stretch: 'both'}
});

// Set the SplitPanel as the only thing in root.

ui.root.widgets().reset([splitPanel]);

Fix the linked map

// -------------------------------------------------------------------------------
//  Work out the linked map. 
// -------------------------------------------------------------------------------

// Center the linked map.  



// Recreate Time 2 image (if we commented it out). 

// var yr2009 = ;

// Display Time 2 image as natural, false color, and accessible false color.

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This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License.