Visualise ESRI ASCII grid

visualise.js

const fs = require('fs/promises')
const { createCanvas } = require('canvas')

function colorScale(p) {
  if (p === 0) return 'white'
  if (p < 50) return '#bad2eb'
  if (p < 250) return '#8ebeda'
  if (p < 500) return '#5a9ecc'
  if (p < 1000) return '#357eb9'
  if (p < 5000) return '#1c5ba6'
  return '#0b3281'
}

async function loadData(file) {
  const content = await fs.readFile(file, 'utf8')
  const data = []

  content.split(/\n+/).forEach((line) => {
    if (/^[0-9-]/.test(line)) {
      const items = line.trim().split(/\s+/)
      data.push(items)
    }
  })

  return data
}

async function draw() {
  // it's only 4.5MB so no need to stream, just bodge!
  const data = await loadData('./UK_residential_population_2011_1_km.asc')

  const canvas = createCanvas(652, 1211)
  const ctx = canvas.getContext('2d')

  data.forEach((row, y) => {
    row.forEach((value, x) => {
      const pop = value === '-9999' ? 0 : parseInt(value, 10)
      ctx.fillStyle = colorScale(pop)
      ctx.fillRect(x, y, 1, 1)
    })
  })

  await fs.writeFile('./map.png', canvas.toBuffer('image/png'))
}

draw()