WMS - Getting data clipped to a polygon

Hello,

The way I set a polygon in my Python OGC requests is by using the custom_url_params argument in my request. You can find more information here.

You will still need to set a bounding box, but you can add your polygon as an optional parameter. Here is a code snippet in Python:

# Set bounding box
bbox_loc = BBox(bbox=[7.3994, 43.7190, 7.4531, 43.7564], crs=CRS.WGS84)

# Polygon WKT (pasted polygon WKT, you can import a file using shapely for example)
geometryWKT = "POLYGON((7.41877555847168 43.72459139811755,7.410106658935547 43.7294293330051,7.414312362670898 43.732158252099964,7.413797378540039 43.734700996616425,7.423152923583985 43.74102638042922,7.432508468627931 43.735693257872555,7.41877555847168 43.72459139811755))"

# WMS request 
request = WmsRequest(data_source=DataSource.SENTINEL2_L1C,
layer='1_TRUE_COLOR',
data_folder=data_folder,
bbox=bbox_loc,
time='latest', 
width=512,
config=config,
custom_url_params={CustomUrlParam.GEOMETRY: geometryWKT}
 )

When making a request for data clipped to a polygon, it’s a good idea to activate the transparency for the areas outside the polygon in the Evalscript that you are calling. Here I am requesting the layer 1_TRUE_COLOR from my dashboard, which has the following evalscript:

//VERSION=3
function setup() {
  return {
    input: ["B02", "B03", "B04", "dataMask"],
    output: { bands: 4 }
  };
}

function evaluatePixel(sample) {
  return [2.5 * sample.B04, 2.5 * sample.B03, 2.5 * sample.B02, sample.dataMask];
}

Thank you very much for taking the time to write such a comprehensive answer, this is exactly what I was looking for, and you explained it very clearly. I hope I can get enough experience to one day give this kind of contribution to the forum.

custom_url_params={CustomUrlParam.GEOMETRY: geometryWKT

Is there an option to use this also inside SentinelHubRequest without using WMS? for example, adding this parameter to the request?

You don’t need this option in SentinelHubRequest, because you can directly specify a Geometry (see options).

The Geometry can be initialised with any of the following geometry representations:

• shapely.geometry.Polygon or shapely.geometry.MultiPolygon
• A GeoJSON dictionary with (multi)polygon coordinates
• A WKT string with (multi)polygon coordinates

e.g.

geometry = Geometry(geometry={"type":"Polygon","coordinates":[[[12.448527,41.92361],[12.480804,41.927186],[12.529563,41.919651],[12.478744,41.922972],[12.448527,41.92361]]]}, crs=CRS.WGS84)

In SentinelHubRequest, how is possible send more than one polygon? I try create a Multipolygon but it seems not to be the way. What is the right way?

Hi,

Sentinelhub supports multipolygons. In the documentation you can see information about the use of a Geometry in SentinelHubRequest:

It currently supports polygons and multipolygons. It can be initialized with any of the following geometry representations:

• shapely.geometry.Polygon or shapely.geometry.MultiPolygon
• A GeoJSON dictionary with (multi)polygon coordinates
• A WKT string with (multi)polygon coordinates

How did you specify the parameter in your request?

That’s a great idea, Maxim. So, how can I do this transparency to NDVI? I’m trying doing as follow but always return a single band

//VERSION=3
function setup() {
  return {
    input: [{
      bands:["B04", "B08", "dataMask"],
    }],
    output: {
      id: "default",
      bands: 2,
    }
  }
}
function evaluatePixel(sample, scene) {
    
    var ndvi = (sample.B08 - sample.B04) / (sample.B08 + sample.B04)
     if (sample.dataMask == 1)  {
    return [ndvi, sample.dataMask ]
  } else {
    return [-1, -1, -1]}
    if (ndvi<-0) return [1.65,0,0.38] 
    else if (ndvi<0.1) return [0.87,0.85,0.61]
    else if (ndvi<0.2) return [0.8,0.78,0.51]
    else if (ndvi<0.3) return [0.74,0.72,0.42]
    else if (ndvi<0.4) return [0.69,0.76,0.38]
    else if (ndvi<0.5) return [0.64,0.8,0.35]
    else if (ndvi<0.6) return [0.57,0.75,0.32]
    else if (ndvi<0.7) return [0.5,0.7,0.28]
    else if (ndvi<0.8) return [0.44,0.64,0.25]
    else if (ndvi<0.9) return [0,1.04,0.55] 
    else return [sample.dataMask]
}

There are a couple of inconsistencies in the evalscript you posted, but these can be easily fixed.

1. The lines starting from if (ndvi<-0) statement are never called because your first if/else statement catches all situations.
2. Your return an array of 2 values in your first if statement, and 3 values in your else, but have set your output bands to 2 in the setup function (see documentation).

It depends what you want to return: the NDVI values or a visualisation of NDVI. If you want to return a visualisation of NDVI with only data showing (where dataMask = 1), you would want to set your output bands to 4 and return 4 values in your evaluatePixel function, where the first three correspond to RGB values, and the last one is the dataMask band (because 0 = noData, and will be interpreted as transparent).