Chapter 23: Data Conversion and Export¶
This chapter provides a workflow to convert raster features to vector and export both raster and vector features for Rocky Mountain National Park, Colorado, United States. The full GEE code can be found here.
Functions¶
/**
* Calculate and add NDSI (snow) band to Landsat 8 image
* @param {ee.Image} image - Landsat 8 image
* @return {ee.Image} - Landsat 8 image with NDSI band added
*/
function add_ndsi(image) {
return image.addBands(image.normalizedDifference(['B3', 'B6']).rename('NDSI'));
}
/**
* Convert raster to vector
* @param {ee.Image} raster Primary or secondary rice field candidates
* @param {ee.FeatureCollection} study_area Study area boundary
* @param {ee.Number} max_pixels Maximum number of pixels to reduce
* @param {ee.Number} raster_scale Scale in meters
* @return {ee.FeatureCollection} Rice fields coverted to vector feature
*/
function raster_to_vector(raster, study_area, max_pixels, raster_scale) {
// Handle optional parameters
max_pixels = max_pixels || 1e7;
raster_scale = raster_scale || 30;
// Convert raster to vector
return raster.reduceToVectors({
geometry: study_area,
maxPixels: max_pixels,
scale: raster_scale
});
}
/**
* Export vector as shapefile to Google Drive or to GEE Asset
* @param {ee.FeatureCollection} vector Vector that will be exported
* @param {string} output_name Name of the output file (without extension)
* @param {string} output_method Name of the export method/location ("Drive" for Google Drive, "Asset" for GEE Asset)
* @return {string} output_message Message indicating the task to run to complete the export
*/
function export_vector(vector, output_name, output_method) {
// Handle optional parameters
output_method = output_method || 'drive';
// Create variable for output message
var output_message;
if (output_method.toLowerCase() == "drive") {
// Export vectors a shapefile to Google Drive
Export.table.toDrive({
collection: vector,
description: output_name,
fileFormat: 'SHP'
});
// Assign output message
output_message = "Task ready: " + output_name + "\nPlease run task to export to Google Drive.";
} else if (output_method.toLowerCase() == "asset") {
// Export vector to GEE Asset
Export.table.toAsset({
collection: vector,
description: output_name,
assetId: output_name
});
// Assign output message
output_message = "Task ready: " + output_name + "\nPlease run task to export to GEE Asset.";
} else {
// Assign output message
output_message = "Invalid export method. Please specify 'Drive' or 'Asset'.";
}
// Return output message
return output_message;
}
/**
* Export image/raster as GeoTiff to Google Drive or to GEE Asset
* @param {ee.Image} image Image/raster that will be exported
* @param {ee.FeatureCollection} study_area Study area boundary
* @param {ee.Number} max_pixels Maximum number of pixels to reduce
* @param {ee.Number} raster_scale Scale in meters
* @param {string} output_name Name of the output file (without extension)
* @param {string} output_method Name of the export method/location ("Drive" for Google Drive, "Asset" for GEE Asset)
* @return {string} output_message Message indicating the task to run to complete the export
*/
function export_raster(raster, study_area, raster_scale, max_pixels, output_name, output_method) {
// Handle optional parameters
output_method = output_method || 'drive';
raster_scale = raster_scale || 30;
max_pixels = max_pixels || 1e7;
// Create variable for output message
var output_message;
if (output_method.toLowerCase() == "drive") {
// Export image/raster as a GeoTiff to Google Drive
Export.image.toDrive({
image: raster,
region: study_area,
scale: raster_scale,
maxPixels: max_pixels,
description: output_name
});
// Assign output message
output_message = "Task ready: " + output_name + "\nPlease run task to export to Google Drive.";
} else if (output_method.toLowerCase() == "asset") {
// Export vector to GEE Asset
Export.image.toAsset({
image: raster,
geometry: study_area,
scale: raster_scale,
maxPixels: max_pixels,
description: output_name,
assetId: output_name
});
// Assign output message
output_message = "Task ready: " + output_name + "\nPlease run task to export to GEE Asset.";
} else {
// Assign output message
output_message = "Invalid export method. Please specify 'Drive' or 'Asset'.";
}
// Return output message
return output_message;
}
Data Acquisition & Preprocessing¶
// Define boundary for Rocky Mountain National Park, Colorado (from GEE Asset)
var rmnp_boundary = ee.FeatureCollection("users/calekochenour/Rocky_Mountain_National_Park__Boundary_Polygon");
// Landsat 8
// Define Landsat 8 collection
var landsat8_t1_sr = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR');
// Filter Landsat 8 Tier 1 SR
var jan_feb = landsat8_t1_sr
.filterDate('2018-01-01', '2018-02-28')
.filterBounds(rmnp_boundary)
.sort('CLOUD_COVER')
.first()
.clip(rmnp_boundary);
// Add NDSI band
jan_feb = add_ndsi(jan_feb);
Data Processing¶
// Extract snow features based on NDSI > 0.5
var jan_feb_snow = jan_feb.select('NDSI').gt(0.5);
var jan_feb_snow_raster = jan_feb_snow.updateMask(jan_feb_snow.eq(1));
// Convert to vector (prepare for export)
var jan_feb_snow_vector = raster_to_vector(jan_feb_snow_raster, rmnp_boundary, 1e8);
Data Postprocessing¶
// No data postprocessing in this lab.
Data Visualization¶
// Define Landsat 8 RGB visualization parameters
var l8_vis_params_rgb = {
'bands': ['B4', 'B3', 'B2'],
'min': 0,
'max': 3000
};
// Define NDSI visualization parameters
var vis_params_ndsi = {
'min': -1,
'max': 1,
palette: ['red', 'green', 'blue', 'white']
};
// Center map to Rocky Mountain National Park, Colorado
Map.setCenter(-105.6836, 40.3428, 10);
// Add RGB image to map
Map.addLayer(jan_feb, l8_vis_params_rgb, 'RMNP - Jan/Feb 2019 - RGB');
// Add NSDI image to map
Map.addLayer(jan_feb.select('NDSI'), vis_params_ndsi, 'RMNP - Jan/Feb 2019 - NDSI');
// Add extracted snow features to map
// Raster
Map.addLayer(jan_feb_snow_raster, {}, 'RMNP - Jan/Feb 2019 - Snow - Raster');
// Vector
Map.addLayer(jan_feb_snow_vector, {color: 'white'}, 'RMNP - Jan/Feb 2019 - Snow - Vector');
Data Export¶
// Export snow features
// Raster
var raster_export = export_raster(
jan_feb_snow_raster,
rmnp_boundary,
undefined, // use default value defined in function
1e8,
'rmnp_snow_jan_feb_raster'
);
// Vector
var vector_export = export_vector(jan_feb_snow_vector, 'rmnp_snow_jan_feb_vector');
// Display export messages
print(raster_export);
print(vector_export);