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December 11, 2015 07:59
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piece of srtm San Francisco area in grayscale png
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| checked in raw |
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| <!doctype html> | |
| <meta charset="utf-8"> | |
| <body> | |
| <script> | |
| var topo, w, h, context, summary; | |
| function setup () { | |
| topo = new Image; | |
| topo.src = "simple_sf.png"; | |
| topo.onload = render; | |
| document.body.appendChild(topo); | |
| } | |
| function render () { | |
| h = topo.height; | |
| w = topo.width; | |
| var canvas = document.createElement("canvas"); | |
| // optional | |
| // document.body.appendChild(canvas); | |
| canvas.width = w; | |
| canvas.height = h; | |
| context = canvas.getContext('2d'); | |
| context.drawImage(topo, 0, 0); | |
| calculate(); | |
| } | |
| function calculate () { | |
| // compare elevation of west half to east half | |
| // counting only px that are on land and >= 1m in | |
| // elevation. no scaling: values are truncated | |
| // to 0-255m to fit in uint8 (affects a few weird | |
| // below sea level measurements and the tippy top of | |
| // the twin peaks) | |
| // I think you could call getImageData on every <i, j> but my try hung | |
| var /* radar */ waves = context.getImageData(0,0,w,h); | |
| var totalEast = 0, | |
| totalWest = 0, | |
| validEast = 0, | |
| validWest = 0, | |
| radar; | |
| for (var j = 0; j < h; j++) { | |
| for (var i = 0; i < w; i++) { | |
| // row column | |
| radar = waves.data[ (j * w * 4) + (i * 4) ]; | |
| if (radar) { | |
| if (i < w/2) { | |
| totalWest += radar; | |
| validWest++; | |
| } | |
| else { | |
| totalEast += radar; | |
| validEast++; | |
| } | |
| } | |
| } | |
| } | |
| summary = [totalWest/validWest, totalEast/validEast]; | |
| reportResult(); | |
| } | |
| function reportResult () { | |
| var result = document.createElement("p") | |
| result.innerHTML = "Average west elevation: " + summary[0].toFixed(1) + " m" + | |
| "<br> Average east elevation: " + summary[1].toFixed(1) + " m"; | |
| document.body.appendChild(result); | |
| } | |
| document.addEventListener("DOMContentLoaded", setup); | |
| </script> |
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