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I am trying to create a correlation chart between NDVI and LST. Everything working fine except the error message saying Error generating chart: FeatureCollection.randomPoints: Polygon too large to be randomly sampled. Must be smaller than a hemisphere. I am also posting the custom script. Please check the script and tell me where I am wrong.

https://code.earthengine.google.com/de9a4508a6bb69365e3bf63cb455b61c

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  • Please add the pertinent code in the body of your question rather than linking to it.
    – Aaron
    Jul 17, 2021 at 14:38

1 Answer 1

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It was just a small issue with the way you were calling in the random points. Try running this now, it should work fine. LINK

//cloud mask
function maskL8sr(col) {
  // Bits 3 and 5 are cloud shadow and cloud, respectively.
  var cloudShadowBitMask = (1 << 3);
  var cloudsBitMask = (1 << 5);
  // Get the pixel QA band.
  var qa = col.select('pixel_qa');
  // Both flags should be set to zero, indicating clear conditions.
  var mask = qa.bitwiseAnd(cloudShadowBitMask).eq(0)
                 .and(qa.bitwiseAnd(cloudsBitMask).eq(0));
  return col.updateMask(mask);
}

//vis params
var vizParams = {
bands: ['B5', 'B6', 'B4'],
min: 0,
max: 4000,
gamma: [1, 0.9, 1.1]
};

var vizParams2 = {
bands: ['B4', 'B3', 'B2'],
min: 0,
max: 3000,
gamma: 1.4,
};

//load the collection:
 {
var col = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
.map(maskL8sr)
.filterDate('2018-01-01','2018-12-31')
.filterBounds(geometry);
}
print(col, 'coleccion');

//imagen reduction
{
var image = col.median();
print(image, 'image');
Map.addLayer(image, vizParams2);
}


//median
{
var ndvi = image.normalizedDifference(['B5', 
'B4']).rename('NDVI');
var ndviParams = {min: -1, max: 1, palette: ['blue', 'white', 
'green']};
print(ndvi,'ndvi');
Map.addLayer(ndvi, ndviParams, 'ndvi');
}

//select thermal band 10(with brightness tempereature), no calculation 
var thermal= image.select('B10').multiply(0.1);
var b10Params = {min: 291.918, max: 302.382, palette: ['blue', 
'white', 'green']};
Map.addLayer(thermal, b10Params, 'thermal');

// find the min and max of NDVI
{
var min = ee.Number(ndvi.reduceRegion({
reducer: ee.Reducer.min(),
geometry: geometry,
scale: 30,
maxPixels: 1e9
}).values().get(0));
print(min, 'min');
var max = ee.Number(ndvi.reduceRegion({
reducer: ee.Reducer.max(),
geometry: geometry,
scale: 30,
maxPixels: 1e9
}).values().get(0));
print(max, 'max')
}

//fractional vegetation
{
var fv =(ndvi.subtract(min).divide(max.subtract(min))).pow(ee.Number(2)).rename('FV'); 
print(fv, 'fv');
Map.addLayer(fv);
}

//Emissivity

var a= ee.Number(0.004);
var b= ee.Number(0.986);
var EM=fv.multiply(a).add(b).rename('EMM');
var imageVisParam3 = {min: 0.9865619146722164, max:0.989699971371314};
Map.addLayer(EM, imageVisParam3,'EMM');

//LST in Celsius Degree bring -273.15
//NB: In Kelvin don't bring -273.15
var LST = thermal.expression(
'(Tb/(1 + (0.00115* (Tb / 1.438))*log(Ep)))-273.15', {
 'Tb': thermal.select('B10'),
'Ep': EM.select('EMM')
}).rename('LST');
Map.addLayer(LST, {min: 20.569706944223423, max:29.328077233404645, palette: [
'040274', '040281', '0502a3', '0502b8', '0502ce', '0502e6',
'0602ff', '235cb1', '307ef3', '269db1', '30c8e2', '32d3ef',
'3be285', '3ff38f', '86e26f', '3ae237', 'b5e22e', 'd6e21f',
'fff705', 'ffd611', 'ffb613', 'ff8b13', 'ff6e08', 'ff500d',
'ff0000', 'de0101', 'c21301', 'a71001', '911003'
 ]},'LST');
 //////////////////////////////////////////////////////////////////////////////
 /////////////////////////////////////////////////////////////////////////////
 // Get the intersection between the two images - the area of interest (aoi).
var aoi2 = ndvi.geometry().intersection(LST.geometry());
// Get a set of 1000 random points from within the aoi. A feature collection
// is returned.

// Create 1000 random points in the region.
var randomPoints = ee.FeatureCollection.randomPoints(geometry);

// Display the points.
Map.centerObject(randomPoints);
Map.addLayer(randomPoints, {}, 'random points');

// Combine the SWIR1 bands from each image into a single image.
var combo = ndvi.select('NDVI')
  .addBands(LST.select('LST'))
  .rename(['ndvi', 'LST']);

// Sample the SWIR1 bands using the sample point feature collection.
var imgSamp = combo.sampleRegions({
  collection: randomPoints,
  scale: 30
})
// Add a constant property to each feature to be used as an independent variable.
.map(function(feature) {
  return feature.set('constant', 1);
});

// Compute linear regression coefficients. numX is 2 because
// there are two independent variables: 'constant' and 's2_swir1'. numY is 1
// because there is a single dependent variable: 'l8_swir1'. Cast the resulting
// object to an ee.Dictionary for easy access to the properties.
var linearRegression = ee.Dictionary(imgSamp.reduceColumns({
  reducer: ee.Reducer.linearRegression({
    numX: 2,
    numY: 1
  }),
  selectors: ['constant', 'ndvi', 'LST']
}));

// Convert the coefficients array to a list.
var coefList = ee.Array(linearRegression.get('coefficients')).toList();

// Extract the y-intercept and slope.
var yInt = ee.List(coefList.get(0)).get(0); // y-intercept
var slope = ee.List(coefList.get(1)).get(0); // slope

// Gather the SWIR1 values from the point sample into a list of lists.
var props = ee.List(['ndvi', 'LST']);
var regressionVarsList = ee.List(imgSamp.reduceColumns({
  reducer: ee.Reducer.toList().repeat(props.size()),
  selectors: props
}).get('list'));

// Convert regression x and y variable lists to an array - used later as input
// to ui.Chart.array.values for generating a scatter plot.
var x = ee.Array(ee.List(regressionVarsList.get(0)));
var y1 = ee.Array(ee.List(regressionVarsList.get(1)));

// Apply the line function defined by the slope and y-intercept of the
// regression to the x variable list to create an array that will represent
// the regression line in the scatter plot.
var y2 = ee.Array(ee.List(regressionVarsList.get(0)).map(function(x) {
  var y = ee.Number(x).multiply(slope).add(yInt);
  return y;
}));

// Concatenate the y variables (Landsat 8 SWIR1 and predicted y) into an array
// for input to ui.Chart.array.values for plotting a scatter plot.
var yArr = ee.Array.cat([y1, y2], 1);

// Make a scatter plot of the two SWIR1 bands for the point sample and include
// the least squares line of best fit through the data.
print(ui.Chart.array.values({
  array: yArr,
  axis: 0,
  xLabels: x})
  .setChartType('ScatterChart')
  .setOptions({
    legend: {position: 'none'},
    hAxis: {'title': 'ndvi'},
    vAxis: {'title': 'LST'},
    series: {
      0: {
        pointSize: 0.2,
        dataOpacity: 0.5,
      },
      1: {
        pointSize: 0,
        lineWidth: 2,
      }
    }
  })
);

output

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