How to select area value from geojson using javascript

JavaScript
1

I’m trying to right a javascript function that selects a polygon based on area value. The criterea is to select largest value if the heighest area value of a feature collection plygons is <= 20. Else, if the smalest area value a feature collection plygons is > 20, to select the smallest value. However, the results I’m getting are not the correct values.

Here is my geojson file



{
  "type": "FeatureCollection",
  "crs": { "type": "name", "properties": { "name": "EPSG:27700" } },
  "features": [
    {
      "type": "Feature",
      "id": 0,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514187.2405629959, 175713.65751711652],
            [514508.69769636169, 175828.21207611449],
            [514376.40576511156, 176106.02513174154],
            [513893.54021604639, 176218.47327330522],
            [513847.23804010917, 176006.80618330464],
            [514191.19706136081, 175887.74344518036],
            [514187.2405629959, 175713.65751711652]
          ]
        ]
      },
      "properties": { "FID": 0, "LSOA21CD": " ", "Area": 17.019149009300001 }
    },
    {
      "type": "Feature",
      "id": 1,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514187.2405629959, 175713.65751711652],
            [514184.58246479835, 175715.76393455453],
            [514182.5480875494, 175714.1718132291],
            [513880.31102292147, 175477.63845830224],
            [514230.88464073557, 175133.6794370506],
            [514535.15608261153, 175437.95087892748],
            [514187.2405629959, 175713.65751711652]
          ]
        ]
      },
      "properties": { "FID": 1, "LSOA21CD": " ", "Area": 18.966876811500001 }
    },
    {
      "type": "Feature",
      "id": 2,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514535.15608261153, 175437.95087892748],
            [514826.1983313635, 175709.14933798835],
            [514508.69769636169, 175828.21207611449],
            [514187.2405629959, 175713.65751711652],
            [514535.15608261153, 175437.95087892748]
          ]
        ]
      },
      "properties": { "FID": 2, "LSOA21CD": " ", "Area": 12.462057224500001 }
    },
    {
      "type": "Feature",
      "id": 3,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514535.15608261153, 175437.95087892748],
            [515004.79243855178, 175186.59620955214],
            [515104.01138698962, 175689.30554830283],
            [514826.1983313635, 175709.14933798835],
            [514535.15608261153, 175437.95087892748]
          ]
        ]
      },
      "properties": { "FID": 3, "LSOA21CD": " ", "Area": 17.1073790847 }
    },
    {
      "type": "Feature",
      "id": 4,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514696.78683049139, 175757.67865081504],
            [514826.1983313635, 175709.14933798835],
            [515104.01138698962, 175689.30554830283],
            [515381.82444261573, 176039.87916611508],
            [515025.50341344811, 176197.6531541571],
            [514696.78683049139, 175757.67865081504]
          ]
        ]
      },
      "properties": { "FID": 4, "LSOA21CD": " ", "Area": 19.0653141355 }
    },
    {
      "type": "Feature",
      "id": 5,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514342.30077446159, 176500.16576545127],
            [514599.64745966345, 176386.21623817086],
            [514918.25281787943, 176786.09847348183],
            [514596.61526418198, 176941.3235537447],
            [514342.30077446159, 176500.16576545127]
          ]
        ]
      },
      "properties": { "FID": 5, "LSOA21CD": " ", "Area": 16.029109829300001 }
    },
    {
      "type": "Feature",
      "id": 6,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514146.01893451158, 176159.67685941607],
            [514376.40576511156, 176106.02513174154],
            [514599.64745966624, 176386.21623817086],
            [514342.30077446159, 176500.16576545127],
            [514146.01893451158, 176159.67685941607]
          ]
        ]
      },
      "properties": { "FID": 6, "LSOA21CD": " ", "Area": 9.3259778883800006 }
    },
    {
      "type": "Feature",
      "id": 7,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514342.30077446159, 176500.16576545127],
            [514052.29053354729, 176628.57826018333],
            [514051.82526298054, 176627.37631121837],
            [513893.54021604639, 176218.47327330522],
            [514146.01893451158, 176159.67685941607],
            [514342.30077446159, 176500.16576545127]
          ]
        ]
      },
      "properties": { "FID": 7, "LSOA21CD": " ", "Area": 11.841352065100001 }
    },
    {
      "type": "Feature",
      "id": 8,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [513976.92131792102, 175961.91581790708],
            [513847.23804010917, 176006.80618330464],
            [513443.74764979444, 175590.08659986407],
            [513619.21886995155, 175544.88946740143],
            [513976.92131792102, 175961.91581790708]
          ]
        ]
      },
      "properties": { "FID": 8, "LSOA21CD": " ", "Area": 8.0748820024299999 }
    },
    {
      "type": "Feature",
      "id": 9,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [513619.21886995155, 175544.88946740143],
            [513880.31102292147, 175477.63845830224],
            [514182.5480875494, 175714.1718132291],
            [514184.58246479835, 175715.76393455453],
            [514187.2405629959, 175713.65751711652],
            [514191.19706136081, 175887.74344518036],
            [513976.92131792102, 175961.91581790708],
            [513619.21886995155, 175544.88946740143]
          ]
        ]
      },
      "properties": { "FID": 9, "LSOA21CD": " ", "Area": 14.7548642798 }
    },
    {
      "type": "Feature",
      "id": 10,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [513619.21886995155, 175544.88946740143],
            [513443.74764979444, 175590.08659986407],
            [513284.99733229354, 175411.49249267764],
            [513532.47008709889, 175269.49992844649],
            [513619.21886995155, 175544.88946740143]
          ]
        ]
      },
      "properties": { "FID": 10, "LSOA21CD": " ", "Area": 5.9491137500200004 }
    },
    {
      "type": "Feature",
      "id": 11,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514731.50091127306, 174880.67981645837],
            [514846.04212105088, 174822.79339861497],
            [515004.79243855178, 175186.59620955214],
            [514906.41315287724, 175239.24991174601],
            [514731.50091127306, 174880.67981645837]
          ]
        ]
      },
      "properties": { "FID": 11, "LSOA21CD": " ", "Area": 4.7672774107500002 }
    },
    {
      "type": "Feature",
      "id": 12,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514906.41315287724, 175239.24991174601],
            [514535.15608261153, 175437.95087892748],
            [514230.88464073557, 175133.6794370506],
            [514731.50091127306, 174880.67981645837],
            [514906.41315287724, 175239.24991174601]
          ]
        ]
      },
      "properties": { "FID": 12, "LSOA21CD": " ", "Area": 19.859035422800002 }
    },
    {
      "type": "Feature",
      "id": 13,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514917.07963621151, 176245.66169233806],
            [515025.50341344532, 176197.6531541571],
            [515320.21211783122, 176592.10942002572],
            [515205.25406012032, 176647.58917831443],
            [514917.07963621151, 176245.66169233806]
          ]
        ]
      },
      "properties": { "FID": 13, "LSOA21CD": " ", "Area": 5.9554811448900002 }
    },
    {
      "type": "Feature",
      "id": 14,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [515205.25406012032, 176647.58917831443],
            [514918.25281787943, 176786.09847348183],
            [514599.64745966345, 176386.21623817086],
            [514917.07963621151, 176245.66169233806],
            [515205.25406012032, 176647.58917831443]
          ]
        ]
      },
      "properties": { "FID": 14, "LSOA21CD": " ", "Area": 16.349272192400001 }
    },
    {
      "type": "Feature",
      "id": 15,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514594.66286209505, 175795.97513896786],
            [514696.78683049139, 175757.67865081504],
            [515025.50341344811, 176197.6531541571],
            [514917.07963621151, 176245.66169233806],
            [514594.66286209505, 175795.97513896786]
          ]
        ]
      },
      "properties": { "FID": 15, "LSOA21CD": " ", "Area": 6.0878052849299999 }
    },
    {
      "type": "Feature",
      "id": 16,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514917.07963621151, 176245.66169233806],
            [514599.64745966624, 176386.21623817086],
            [514376.40576511156, 176106.02513174154],
            [514508.69769636169, 175828.21207611449],
            [514594.66286209505, 175795.97513896786],
            [514917.07963621151, 176245.66169233806]
          ]
        ]
      },
      "properties": { "FID": 16, "LSOA21CD": " ", "Area": 16.9024733312 }
    },
    {
      "type": "Feature",
      "id": 17,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514040.11921087652, 176716.00784859248],
            [514051.82526298054, 176627.37631121837],
            [514052.29053354729, 176628.57826018333],
            [514342.30077446159, 176500.16576545127],
            [514596.61526418198, 176941.3235537447],
            [514294.3159738658, 177087.21581993997],
            [514040.11921087652, 176716.00784859248]
          ]
        ]
      },
      "properties": { "FID": 17, "LSOA21CD": " ", "Area": 18.067530070499998 }
    },
    {
      "type": "Feature",
      "id": 18,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [515631.83968934324, 176441.71524421126],
            [515320.21211783122, 176592.10942002572],
            [515025.50341344532, 176197.6531541571],
            [515381.82444261573, 176039.87916611508],
            [515640.5562169617, 176267.38469183631],
            [515631.83968934324, 176441.71524421126]
          ]
        ]
      },
      "properties": { "FID": 18, "LSOA21CD": " ", "Area": 19.848118984599999 }
    },
    {
      "type": "Feature",
      "id": 19,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [514154.5027258927, 175031.8368839249],
            [514230.88464073557, 175133.6794370506],
            [513880.31102292147, 175477.63845830224],
            [513619.21886995155, 175544.88946740143],
            [513532.47008709889, 175269.49992844649],
            [513897.89380010497, 175059.83058492094],
            [514154.5027258927, 175031.8368839249]
          ]
        ]
      },
      "properties": { "FID": 19, "LSOA21CD": " ", "Area": 19.771145470800001 }
    }
  ]
}

And here is the javascript function that I wrote.

const data = require("./area.json");

console.log(
  data.features.reduce((sampleArea, selectedArea) => {
    if (sampleArea[selectedArea.properties.Area] <= 20) {
      Math.max(sampleArea[selectedArea.properties.Area]);
    } else Math.min(sampleArea[selectedArea.properties.Area]);
    return selectedArea.properties.Area;
  }, {})
);

The answer I got is

PS E:\polygons\test> node script.js 19.7711454708

However the correct answer is

19.859035422800002

I have tried with a different geojson data, where the smallest area is 23.884209, but I got 35.5510673211

2

Hey @JohnH08, first we should clarify what your goal is. Are you trying to find a single match or multiple matches based on your area criteria?

If you’re trying to find any matches, .reduce() is not the method to use. .reduce() is meant to reduce a set of items to one single output, typically with an accumulator.

If you all want is a match/matches, you want to use the following: If you want to only get the first match found, you can use .find() and if you want multiple matches, you can use a method such as .filter().

3

Thanks for your quick reply @marcusparsons. I want to find a single match.

4

Do you want to find the feature that contains the minimum of all Area properties? Or just the value of that Area? Your very first feature is the one that will come back if you use .find() for a single match for an Area less than 20. But it’s a little bit of a different story to get the minimum one.

5

The value I want to select is one polygon everytime I run the function. When I get the data, I want the function to select the polygon with area <= 20. If there is a polygon with area value of 20, It is the best choice, if not any value closer to 20 can do. However, if the smallest value of the data is > 20, I want to select the smallest value.
For example, if I use the JSON file below, want to select the polygon with area value of 23.884209.

{
  "type": "FeatureCollection",
  "crs": { "type": "name", "properties": { "name": "EPSG:27700" } },
  "features": [
    {
      "type": "Feature",
      "id": 0,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [511116.20154187363, 175591.38891189359],
            [510834.16784317698, 176081.48025717214],
            [510310.01419806667, 175638.14761470631],
            [510545.3343814835, 175302.48427530564],
            [511116.20154187363, 175591.38891189359]
          ]
        ]
      },
      "properties": {
        "FID": 0,
        "LSOA21CD": "E01002589",
        "Area": 32.076108972699998
      }
    },
    {
      "type": "Feature",
      "id": 1,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [510783.73012447171, 174962.43392336369],
            [511065.19299874175, 174560.95213407092],
            [511670.86817863118, 174855.8943842072],
            [511670.66317971516, 174856.27312230133],
            [511481.41056893207, 175205.91971606016],
            [510783.73012447171, 174962.43392336369]
          ]
        ]
      },
      "properties": {
        "FID": 1,
        "LSOA21CD": "E01002589",
        "Area": 30.825945690000001
      }
    },
    {
      "type": "Feature",
      "id": 2,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [511481.41056893207, 175205.91971606016],
            [511239.10261550453, 175653.58672686294],
            [510545.3343814835, 175302.48427530564],
            [510783.73012447171, 174962.43392336369],
            [511481.41056893207, 175205.91971606016]
          ]
        ]
      },
      "properties": { "FID": 2, "LSOA21CD": "E01002589", "Area": 34.5472262567 }
    },
    {
      "type": "Feature",
      "id": 3,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [509921.09005591739, 174661.3783289697],
            [510037.89187975135, 174415.33425035886],
            [511065.19299874175, 174560.95213407092],
            [510783.73012447171, 174962.43392336369],
            [509921.09005591739, 174661.3783289697]
          ]
        ]
      },
      "properties": { "FID": 3, "LSOA21CD": " ", "Area": 35.042002141499999 }
    },
    {
      "type": "Feature",
      "id": 4,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [510783.73012447171, 174962.43392336369],
            [510545.3343814835, 175302.48427530564],
            [509695.40423042234, 175136.78752350435],
            [509921.09005591739, 174661.3783289697],
            [510783.73012447171, 174962.43392336369]
          ]
        ]
      },
      "properties": { "FID": 4, "LSOA21CD": " ", "Area": 40.328572387800001 }
    },
    {
      "type": "Feature",
      "id": 5,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [512255.23007169273, 176192.46413836256],
            [512704.89721376076, 174928.62694361061],
            [512879.52256301232, 174928.62694361061],
            [512509.1051555099, 176516.13011861779],
            [512255.23007169273, 176192.46413836256]
          ]
        ]
      },
      "properties": { "FID": 5, "LSOA21CD": " ", "Area": 37.180851322400002 }
    },
    {
      "type": "Feature",
      "id": 6,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [511535.81214587204, 176132.41452069953],
            [512006.39581675828, 176891.83920336887],
            [511032.72720275354, 176987.08939386904],
            [510972.8317725556, 176891.67279308103],
            [511237.44918055646, 176422.57829708047],
            [511467.8129753042, 176022.67792819999],
            [511535.81214587204, 176132.41452069953]
          ]
        ]
      },
      "properties": { "FID": 6, "LSOA21CD": " ", "Area": 49.954760330200003 }
    },
    {
      "type": "Feature",
      "id": 7,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [511824.04491915181, 175642.74510737322],
            [511535.81214587204, 176132.41452069953],
            [511239.10261550453, 175653.58672686294],
            [511481.41056893207, 175205.91971606016],
            [511824.04491915181, 175642.74510737322]
          ]
        ]
      },
      "properties": { "FID": 7, "LSOA21CD": " ", "Area": 27.126790326999998 }
    },
    {
      "type": "Feature",
      "id": 8,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [511824.04491915181, 175642.74510737322],
            [512255.23007169273, 176192.46413836256],
            [512006.39581675828, 176891.83920336887],
            [511535.81214587204, 176132.41452069953],
            [511824.04491915181, 175642.74510737322]
          ]
        ]
      },
      "properties": { "FID": 8, "LSOA21CD": " ", "Area": 44.383529100700002 }
    },
    {
      "type": "Feature",
      "id": 9,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [512262.6076927837, 174897.68487835862],
            [511824.04491915181, 175642.74510737322],
            [511481.41056893207, 175205.91971606016],
            [511670.66317971516, 174856.27312230133],
            [512262.6076927837, 174897.68487835862]
          ]
        ]
      },
      "properties": { "FID": 9, "LSOA21CD": " ", "Area": 33.083362414500002 }
    },
    {
      "type": "Feature",
      "id": 10,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [512262.6076927837, 174897.68487835862],
            [512704.89721376076, 174928.62694361061],
            [512255.23007169273, 176192.46413836256],
            [511824.04491915181, 175642.74510737322],
            [512262.6076927837, 174897.68487835862]
          ]
        ]
      },
      "properties": { "FID": 10, "LSOA21CD": " ", "Area": 56.762039448000003 }
    },
    {
      "type": "Feature",
      "id": 11,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [510834.16784317698, 176081.48025717214],
            [510619.92045149952, 176453.77900336869],
            [510022.6422551889, 176048.05816549063],
            [510023.86025670078, 176046.32079452462],
            [510310.01419806667, 175638.14761470631],
            [510834.16784317698, 176081.48025717214]
          ]
        ]
      },
      "properties": {
        "FID": 11,
        "LSOA21CD": "E01002589",
        "Area": 32.577401352499997
      }
    },
    {
      "type": "Feature",
      "id": 12,
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [509515.79092483688, 175515.14454592392],
            [509695.40423042234, 175136.78752350435],
            [510545.3343814835, 175302.48427530564],
            [510310.01419806667, 175638.14761470631],
            [509515.79092483688, 175515.14454592392]
          ]
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      },
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6

Here’s what I did to solve this problem:

  • I used filter() to go through every feature in the GeoJSON data.
  • I tested each Area property against a given value combined with a marginOfError (this is where you would decide how close to exact you want to match to a given value i.e. 20)
  • If a property fit in the criteria, I set the feature and area to their own variables and return them.
7

The first problem I see is that you are not returning anything into the reduce callback but the selectedArea value. In fact the result u got is the last one in the json data.

You should return a value in the if else statement

8

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