Extracting LANDSAT time-series data by polygon using Google Earth Engine?

Question

importing the landsat 5 TM 32-day NDVI composite data set.

I have made it as far as drawing a polygon of my ROI and extracting from a different data set, a global land cover map (ESA/GLOBCOVER_L4_200901_200912_V2_3) which was linked from another question posted on here:

Drawing polygon and extracting land cover inside using Google Earth Engine?

I tried to reconfigure it to work with my Landsat NDVI set and I was met with errors. I tried to extract just the NDVI band and try with that, but I keep getting an error:

land.clip is not a function in , line #

var landsat: Imagecollection "Landsat 5 TM 32-Day NDVI Composite"  
var geometry: Polygon, 4 vertices  
var globcover: Image "GlobCover: GLobal Land Cover Map"

// Extract the landcover band    
var land = globcover.select('landcover');    
// Clip the image to the polygon geometry    
var landcover_roi = land.clip(geometry);  
// Add a map layer of the landcover clipped to the polygon.  
Map.addLayer(landcover_roi);

reworking to accomodate the landsat:

// Extract the landcover band  
    var land = landsat.select('NDVI');  
    // Clip the image to the polygon geometry  
    var landcover_roi = land.clip(geometry);  
    // Add a map layer of the landcover clipped to the polygon.  
    Map.addLayer(landcover_roi);

This is where I get the error,

land.clip is not a function in <global> line 4.

I am attempting to draw a polygon (square) around an area on the LS5NDVI data, export the "data cube", and correlate the time NDVI values at each pixel against various climate data to produce a single layer "heat map" of sorts to visually represent the correlation at each pixel.

Answer 1

The code that you copied uses an Earth Engine object of type "Image", and Image objects have a method/function named clip. Your reworked code (i.e. Landsat) uses an Earth Engine object of type "ImageCollection", and ImageCollection objects do not have a method named clip, so it produces an error. To get around this, you can either:

1. Convert the ImageCollection to an Image using a reducing method (i.e. median, mode, mosaic, max, etc.)
2. Write a function that clips an Image object, and map that function over the collection using ee.ImageCollection.map().

Also, it probably doesn't make sense to add the entire Landsat 5 collection as a layer on the interactive map... it contains over 25 years of data. Add a step that filters the Landsat collection down to a shorter time interval, using ImageCollection.filterDate().