27:null 28:["$","title",null,{"children":"GeoTIFF contours II | Catherine Deal's Workspace | Observable"}] 2d:[{"id":"6954c8d84aa4a9b4de4dfcc4cbe86556571ecdcbbac8f587cc98207c9a613b03ad260859f796a61b2facf50b50038f113c8cd5f55c4fbe1e2c1bc785e8ac3805","href":"https://static.observableusercontent.com/files/6954c8d84aa4a9b4de4dfcc4cbe86556571ecdcbbac8f587cc98207c9a613b03ad260859f796a61b2facf50b50038f113c8cd5f55c4fbe1e2c1bc785e8ac3805","name":"sfctmp.tiff","type":"image/tiff","lastModified":1698786254444.482,"size":2083493}] 2b:{"id":"baaa3f15bd59876e","slug":null,"title":"GeoTIFF contours II","thumbnail":"3b17e99c8f8c562e95165d1a9253dc142609f5b55b207637abd0fdd1914cc3f9","access_level":"public","create_time":"$D2023-10-31T21:04:14.444Z","update_time":"$D2023-10-31T21:04:14.444Z","view_time":null,"first_public_time":"$D2023-10-31T21:04:14.444Z","has_unpublished_edits":null,"fork":{"id":"6ac0a38a2ff5829c","slug":"geotiff-contours-ii","title":"GeoTIFF contours II","owner":{"id":"863951e3ebe4c0ae","login":"d3","name":"D3"}},"stars":0,"starred":false,"owner":{"type":"team","id":"f78043483f179305","login":"catherine-deal-ws","name":"Catherine Deal's Workspace","avatar_url":"https://avatars-next.observableusercontent.com/4ec4a3f2b854476d7515e919a7797cce5de900ff54ad61af93e94e1d4bea24c3","tier":"starter_2024"},"version":146,"publish_version":null,"role":null,"authored":false,"authors":[{"type":"individual","id":"f2ee2f1935931d21","login":"catherine-deal","name":"Catherine Deal","avatar_url":"https://avatars-next.observableusercontent.com/bf2916f2279ef122e5fe796d142137e4d94f582b39b634b55f4526e7964b4374","tier":null}],"members":[{"type":"individual","id":"f2ee2f1935931d21","login":"catherine-deal","name":"Catherine Deal","avatar_url":"https://avatars-next.observableusercontent.com/bf2916f2279ef122e5fe796d142137e4d94f582b39b634b55f4526e7964b4374","tier":null,"role":"editor"},{"type":"team","id":"f78043483f179305","login":"catherine-deal-ws","name":"Catherine Deal's Workspace","avatar_url":"https://avatars-next.observableusercontent.com/4ec4a3f2b854476d7515e919a7797cce5de900ff54ad61af93e94e1d4bea24c3","tier":"starter_2024","role":"editor"}],"comments":[],"custom_thumbnail":false,"trashed":false,"lapsed":null,"subscribed":null,"forks":0,"collection":null,"type":"notebook","body":{"title":"GeoTIFF contours II","cells":[{"id":0,"value":"

GeoTIFF contours II

D3 › Gallery

\n\n# GeoTIFF contours II\n\nThese contours are computed in equirectangular coordinates and then reprojected.","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":31,"value":"chart = html``","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":38,"value":"projection = d3.geoNaturalEarth1().precision(0.1)","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":33,"value":"path = d3.geoPath(projection)","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":29,"value":"contours = d3.contours().size([n, m])","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":27,"value":"color = d3.scaleSequential(d3.extent(values), d3.interpolateMagma)","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":16,"value":"image = tiff.getImage()","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":15,"value":"values = rotate((await image.readRasters())[0])","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":24,"value":"n = image.getWidth()","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":22,"value":"m = image.getHeight()","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":47,"value":"The *rotate* function rotates a GeoTIFF’s longitude from ${tex`[0, 360]`} to ${tex`[-180, +180]`}.","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":42,"value":"function rotate(values) {\n var l = n >> 1;\n for (var j = 0, k = 0; j < m; ++j, k += n) {\n values.subarray(k, k + l).reverse();\n values.subarray(k + l, k + n).reverse();\n values.subarray(k, k + n).reverse();\n }\n return values;\n}","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":50,"value":"The *invert* function converts [*x*, *y*] in pixel coordinates to [*longitude*, *latitude*]. This assumes the source GeoTIFF is in equirectangular coordinates.\n\nInverting the projection breaks the polygon ring associations: holes are no longer inside their exterior rings. Fortunately, since the winding order of the rings is consistent and we’re now in spherical coordinates, we can just merge everything into a single polygon!","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":45,"value":"function invert(d) {\n const shared = {};\n\n let p = {\n type: \"Polygon\",\n coordinates: d3.merge(d.coordinates.map(polygon => {\n return polygon.map(ring => {\n return ring.map(point => {\n return [point[0] / n * 360 - 180, 90 - point[1] / m * 180];\n }).reverse();\n });\n }))\n };\n\n // Record the y-intersections with the antimeridian.\n p.coordinates.forEach(ring => {\n ring.forEach(p => {\n if (p[0] === -180) shared[p[1]] |= 1;\n else if (p[0] === 180) shared[p[1]] |= 2;\n });\n });\n\n // Offset any unshared antimeridian points to prevent their stitching.\n p.coordinates.forEach(ring => {\n ring.forEach(p => {\n if ((p[0] === -180 || p[0] === 180) && shared[p[1]] !== 3) {\n p[0] = p[0] === -180 ? -179.9995 : 179.9995;\n }\n });\n });\n\n p = d3.geoStitch(p);\n\n // If the MultiPolygon is empty, treat it as the Sphere.\n return p.coordinates.length\n ? {type: \"Polygon\", coordinates: p.coordinates}\n : {type: \"Sphere\"};\n}","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":13,"value":"tiff = FileAttachment(\"sfctmp.tiff\").arrayBuffer()\n .then(buffer => GeoTIFF.fromArrayBuffer(buffer))","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":6,"value":"GeoTIFF = require(\"geotiff@2.0.7\")","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":5,"value":"d3 = require(\"d3@7\", \"d3-geo-projection@4\")","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":91,"value":"Or, using [Observable Plot](/plot/)’s concise API:","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":94,"value":"Plot.plot({\n projection: \"equal-earth\",\n color: { scheme: \"Magma\" },\n marks: [Plot.contour(values, {\n x: (_, i) => i % n / 2 - 180,\n y: (_, i) => 90 - Math.floor(i / n) / 2,\n fill: values,\n thresholds: 10,\n stroke: \"#000\",\n strokeWidth: 0.5,\n clip: \"sphere\"\n })]\n})","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"}],"files":"$W2d","stdlib":"1","titling":"auto"}} 2c:"$2b:body"