Geospatial_analysis_Yale-NUS.md

Geospatial Software Analysis - An Introduction to Google Earth Engine

TC Chakraborty

Yale School of Forestry & Environmental Studies

Yale Data Driven Solutions Group

March 16, 2018

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https://github.com/datadrivenyale/day-of-data-2.0

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Agenda

Introduction to Geospatial Analysis

Data Types

The Google Earth Engine platform

Basic functions

Variable types

Geometry declaration

Functions and mapping

Operations on Geometries

Features and Feature Collections

Operations on Features

Filters

Operations on Images

Operations on Image Collections

Importing and exporting Data

Conclusion and resources

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Introduction

Collection, visualization, and analysis of geographical or spatial data.

Data types

• Vector data represent lat-long coordinates

• Raster data comprises of pixels with associated values

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• Points

• Lines

• Polygons

• Raster layers/bands

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Google Earth Engine platform

Code Editor

• Cloud-based platform for planetary scale geospatial analysis
• Uses Google's computational resources to reduce processing time
• Massive archive of remote sensing data
• 200 public datasets
• greater than 4000 new images every day
• greater than 5 million images
• greater than 5 petabytes of data

Source: Google Earth Engine User summit

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Basic Functions

Declaring variables

var varname = Containerforvariabletype(variable name); 

Centering map

Map.setCenter(long, lat, zoom level);

Zoom level varies from 0 (no zoom) to 20 (highest zoom level)

Displaying metadata

print(variable name) 

Adding a layer to the map

Map.addLayer(VARIABLENAME);

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Variable types in Earth Engine

Strings

var var_String = ee.String("This is a string. Or is it? It is."); 

Numbers

var var_Numbers = ee.Number(5);

Arrays

var var_Array = ee.Array([[5, 2, 3],  [-2, 7, 10],  [6, 6, 9]]); 

Lists

var var_List = ee.List([5, "five" , 6, "six"]); 

Dictionaries

var var_Dictionary = ee.Dictionary({five: 5 , six: 6}); 

And the fun stuff

• Geometries
• Features
• Feature Collections
• Images
• Image Collections

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Geometries – declaration and types

Points

var var_Point = ee.Geometry.Point(0, 45);

Multi Points

var var_MultiPoint = ee.Geometry.MultiPoint(0, 45, 5,6, 70,-56);

Line String

var var_LineString = ee.Geometry.LineString([[0, 45], [5,6], [70,-56]]);

Multi Line String

var var_MultiLineString = ee.Geometry.MultiLineString([[[0, 45], [5,6], [70,-56]], [[0, -45], [-5,-6], [-70,56]]]);

Linear Ring

var var_LinearRing = ee.Geometry.LinearRing(0, 45, 5,6, 70,-56, 0,45);

Rectangle

var var_Rectangle = ee.Geometry.Rectangle(0, 0, 60,30);

Polygon

var var_Polygon = ee.Geometry.Polygon([[[0, 0], [6,3], [5, 5], [-30,2], [0,0]]]);

Multi Polygon

var var_MultiPolygon = ee.Geometry.MultiPolygon([ee.Geometry.Polygon([[0, 0], [6, 3], [5, 5], [-30, 2], [0,0]]), ee.Geometry.Polygon([[0, 0], [-6, -3], [-5, -5], [30, -2], [0, 0]])]);

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Features and Feature Collections

• Features are geometries associated with specific properties
• Feature Collections are groups of features

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Functions and mapping

• A set of instructions to perform a specific task

function  function_Name(Arguments) {statements};

Call function

var result = function_Name(Input);

Map function over Feature or Image Collection

var result = Input.map(function_Name);

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Operations on Geometries

Geometry operations

Find area of geometry

var Geometry_area = var_Geometry.area();

Find length of line

var Line_length = var_LineString.length();

Find perimeter of geometry

var Geometry_perimeter = var_Geometry.perimeter();

Reduce number of vertices in geometry

var SimplifiedGeometry = var_Geometry.simplify(100);

Find centroid of geometry

var Centroid = var_Geometry.centroid();

Create buffer around geometry

var Buffer = var_Geometry.buffer(100);

Find bounded rectangle of the Geometry

var BoundedGeometry = var_Geometry.bounds();

Find the smallest envelope that can envelop the Geometry

var Convexhull_Geometry = var_Geometry.convexHull();

Find common area between two or more geometries

var Inter_geometry = var_Geometry1.intersection(var_Geometry2);

Find area that includes two or more geometries

var Union_geometry = var_Geometry1.union(var_Geometry2);

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Operations on Features

Feature operations

Set property name and value of geometry to create a feature

var var_Feature = ee.Feature(var_Geometry, {Name: "Feature name", Size: 500};

Create a new feature from existing feature while renaming a property

var var_Featurenew = var_Feature.select(["Name"], ["Descriptor"]);

Extract values of a property from a Feature

var values = var_Feature.get(''Size'');

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Filters

Creator a filter for values of a property

var BFilter = ee.Filter.eq(Property_name, Value);

or .neq , .gt , .gte , .lt , and .lte

Create a filter based on maximum difference from a threshold

var DiffFilter = ee.Filter.maxDifference(threshold, Property_name, Value);

Create a text filter

var TxtFilter = ee.Filter.stringContains( Property_name, StringValue);  

or .stringStartsWith, and .stringEndsWith

Create a range filter

var RangeFilter = ee.Filter.rangeContains( Property_name, StringValue, MinValue, MaxValue);

Create a list filter to check for certain values

var ListFilter = ee.Filter.listContains(Property_name, Value1, Property_name2, Value2); 

.inList to test against list of values

Create a filter of dates

var DateFilter = ee.Filter.calendarRange(StartDate, StopDate);

Create a filter for particular days of the year

var DayFilter = ee.Filter.dayOfYear(startDay, StopDay);

Create a filter to subset geospatial data

var BoundsFilter= ee.Filter.bounds(GeometryorFeature);

Combining and inversing filters

var NewFilter=ee.Filter.and(Listoffilters);
var NewFilter=ee.Filter.or(Listoffilters);
var inverseFilter = ee.Filter.not(filter);

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Operations on Images

Image operations

Selecting the bands of an image

var band = var_Image.select(band name);

Creating masks

var mask =var_Image.eq(value); 

or .neq or .gt or .gte or .lt or .lte

Applying masks

var masked =var_Image.mask(mask);

Pixelwise calculation

var results =var_Image.sum(value); 

or .subtract , .multiply , .divide , .max , .min , .abs , .round , .floor , .ceil , .sqrt , .exp, .log, .log10, .sin , .cos , .tan , .sinh , .cosh , .tanh , .acos, .asin

Shift pixels of an image

newImage = oldImage.leftShift(valueofshift);                       

or .rightShift

Create a single value from an image by applying a reducer based on regions of interest

var outputDictionary = var_Image.reduceRegion(Reducer, var_Geometry, scale);

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Operations on Image Collections

Select the first n numbers of images in a collection (based on property)

var SelectedImages =var_ImCollection.limit (n, Property_name, Order);

Select images in collection based on particular properties

var SelectedImages = var_ImCollection.filterMetadata (Property_name, Relation , Value);

Relations could be "equals", "less_than", "greater_than", "starts_with", "ends_with", and "contains"

Select images within date range

var SelectedImages = var_ImCollection.filterDate (StartDate, StopDate);

Select images within Geometry

var SelectedImages = var_ImCollection.filterBounds (var_Geometry);

Perform pixelwise calculations for all images in collection

var sumofimages = var_ImCollection.sum();       

or .product, .max, .min, .mean, .mode, .median, and .count

Create composite of images in collection with the last image on top

var mosaicofimages = var_ImCollection.mosaic();       

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Importing and exporting data

Image to table example

Timelapse example

Export image, video or table to Google Drive, Asset, or Google Cloud

Export.image.toDrive({
  collection: var_Image, description: 'FileName', region: var_Geometry, scale: 1000}
});

or image.toCloudStorage, image.toAsset, table.toDrive, table.toCloudStorage, video.toCloudStorage, and video.toDrive

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Conclusion and resource

Night Lights example

• Adapted from Prof. Dana Tomlin's notes

Resources

Google Earth Engine API documentation

Google Earth Engine Developers forum

Example scripts from Prof. Dana Tomlin's handouts for his course on Geospatial Software Design

• [email protected]