2b:null 2c:["$","title",null,{"children":"Générateur de courbes de niveau | Atelier de cartographie de Sciences Po | Observable"}] 31:T6fa,## Un double objectif 1. Transformer des données d'élévation au format raster en des isolignes sous forme de polygones au format geojson pour des niveaux d'altitude définis. 2. Représenter ses isolignes (polygones) à la manière de contours illuminés Tanaka. ## Source des données de relief [Terrain Tile](https://registry.opendata.aws/terrain-tiles/) de Mapzen propose des données d'élévation sous forme de tuile raster à différent niveaux de zoom. Pour les besoins de Graticule ces fichiers doivent être retravaillés pour respecter des prérequis : + Avoir un fichier raster par résolution NaturalEarth. Soit les zoom 0, 1 et 2 de Terrain Tiles pour les résolutions 110m, 50m et 10m de NaturalEarth + Conserver des valeurs pour les seules terres émergées + Être le plus léger possible (poids des fichiers) ### Préparation amont des raster (avec [GDAL](https://gdal.org/)) 1. fusionner les tuiles de chacun des trois niveaux en un seul raster tif 2. conserver les données pour les terres (pas les océans) en découpant chacun des trois raster fusionné selon la couche NaturalEarth land 10m + un buffer (_le véritable clip qui associe la bonne résolution NaturalEarth au bon niveau de zoom Terrain Tiles se fera à la volé en svg dans le navigateur_) 3. reprojeter les raster, de EPSG:3857 (pseudo-Mercator) en EPSG:4326 (WGS 84) 4. convertir les valeurs en entier au lieu de nombre à virgule 5. arrondir les valeurs à la dizaine (ex: 43 => 40, 245 => 250, 1268 => 1270) 6. compresser le résultat, cf https://kokoalberti.com/articles/geotiff-compression-optimization-guide/ Le résultat en terme de poids de fichier est le suivant (_étape 1. versus étape 6._) : zoom 0 (_450ko => 128ko_) zoom 1 (_3,2Mo => 432ko_) zoom 2 (_13Mo => 1,4Mo_) 32:T568,## Représentation du relief Parmi les [nombreuses techniques de représentation du relief](https://en.wikipedia.org/wiki/Terrain_cartography), le choix pour l'outil Graticule c'est orienté vers les [contours illuminés Tanaka](http://wiki.gis.com/wiki/index.php/Tanaka_contours). Essentiellement car Graticule donne la liberté à l'utilisateur de choisir une projection et de la paramétrer. Une technique comme Tanaka, qui peut être mise en oeuvre simplement et dynamiquement dans le navigateur, prend tout son sens. La représentation du relief devient alors indépendante de la projection et du zoom de la carte. En simplifiant largement cette technique, on peut la résumer à associer chaque isoligne à deux ombrages ayant des directions opposées. L'un clair, l'autre foncé. Pour créer ces ombrages, deux méthodes sont possibles : le filtre svg [`feDropShadow`](https://developer.mozilla.org/fr/docs/Web/SVG/Element/feDropShadow) ou le filtre css [`drop-shadow`](https://developer.mozilla.org/fr/docs/Web/CSS/filter-function/drop-shadow). On peut voir la comparaison des deux ci-dessous. Bien que le résultat semble identique, le filtre css présente deux défauts inexpliqués : 1. la jonction entre les deux ombrages n'est pas parfaite, un vide apparait parfois et 2. l'épaisseur des ombrages opposés n'est pas toujours le même selon la direction de la lumière.33:T591,34:T4d9,addMap = (proj, zoom = false, dimensions = [width, 400]) => { const id = d3.randomInt(100)() const outline = ({type: "Sphere"}) const path = d3.geoPath(proj.fitSize(dimensions, outline)) const map = htl.svg `` // Pan & Zoom const addZoom = d3 .zoom() .scaleExtent([1, 20]) // min, max du zoom .translateExtent([[0, 0], dimensions]) // bornes extérieures du translate .on("zoom", ({ transform }) => { d3.select(map).select("g#zoom").attr("transform", transform) mutable k = transform.k }) if (zoom) d3.select(map).call(addZoom) return map 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Ansart","avatar_url":"https://avatars-next.observableusercontent.com/b086dcc95470aae08b776435f7604190c606884a2187354d54b5fbae116aa241","tier":null}],"members":[{"type":"team","id":"5d76df5eab7f8bb7","login":"ateliercartographie","name":"Atelier de cartographie de Sciences Po","avatar_url":"https://avatars-next.observableusercontent.com/fab2304a50866da3a682fdbbdf200ecb01ddfd5fbf74f60756d36188cfd2cf68","tier":"starter_2024","role":"editor"},{"type":"individual","id":"b5f87f2a35829281","login":"tombor","name":"Thomas Ansart","avatar_url":"https://avatars-next.observableusercontent.com/b086dcc95470aae08b776435f7604190c606884a2187354d54b5fbae116aa241","tier":null,"role":"editor"}],"comments":[],"custom_thumbnail":false,"trashed":false,"lapsed":null,"subscribed":null,"forks":2,"collection":{"id":"ae5edacf60f09e75","slug":"graticule","title":"Graticule","owner":{"id":"5d76df5eab7f8bb7","login":"ateliercartographie","name":"Atelier de cartographie de Sciences Po"}},"type":"notebook","body":{"title":"Générateur de courbes de niveau","cells":[{"id":0,"value":"# Générateur de courbes de niveau\n### Au format geojson à partir d'un modèle numérique du terrain au format tif\n> Ce travail est utilisé dans [**Graticule**](https://www.sciencespo.fr/cartographie/graticule/), un outil conçu et développé par l’Atelier de cartographie de Sciences Po. Gratuit et open source, il permet à tout type de public de s’initier à la cartographie et ses concepts par la réalisation de fond de carte.","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":329,"value":"toc(\"h2\", 'Plan')","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":196,"value":"$31","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":72,"value":"rawTiff = {\n switch (basemapRes) {\n case '110m': return FileAttachment(\"z0@1.tif\")\n case '50m' : return FileAttachment(\"z1@1.tif\")\n case '10m' : return FileAttachment(\"z2@1.tif\")\n }\n} ","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":260,"value":"## Source du fond de carte\nUtilisation d'une version modifiée de NaturalEarth v5, d'après le notebook [@tombor/adapter-natural-earth-les-pays](https://observablehq.com/@tombor/adapter-natural-earth-les-pays).","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":67,"value":"rawCountries = {\n switch (basemapRes) {\n case '110m': return FileAttachment(\"countries_110m.json\").json()\n case '50m' : return FileAttachment(\"countries_50m.json\").json()\n case '10m' : return FileAttachment(\"countries_10m.json\").json()\n }\n} ","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":265,"value":"## Du raster au geojson\nDeux approches permettent d'arriver au résultat attendu. La première consiste à faire la transformation en amont à l'aide d'outil comme GDAL ou QGIS. On génère les isolignes de chaque niveau d'élévation nécessaire. Pour les besoins de Graticule, cette approche présente deux défauts : 1. la somme du poids des geojson générés est plus lourde que les raster compressés en entrée et 2. si on veut laisser le choix à l'utilisateur des niveaux d'élévation qu'il souhaite afficher, ce n'est pas possible à moins de pré-générer encore plus de niveaux d'élévation.\n\nLa seconde approche consiste à effectuer la transformation dans le navigateur et à simplement charger les raster. Moins de données transférées et l'utilisateur pourra générer/afficher n'importe quel niveau de relief. \n**Grandes étapes :**\n+ lire les valeurs du raster\n+ créer les isolignes\n+ géoréférencer les isolignes","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":65,"value":"### Lecture des valeurs du raster\n_À l'aide de la librairie [geotiff.js](https://github.com/geotiffjs/geotiff.js)_","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":10,"value":"tiff = rawTiff.arrayBuffer() // valable pour les formats binaire\n .then(buffer => geotiff.fromArrayBuffer(buffer))","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":31,"value":"image = tiff.getImage()","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":33,"value":"values = (await image.readRasters())[0]","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":35,"value":"n = image.getWidth() // largeur en pixel","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":37,"value":"m = image.getHeight() // hauteur en pixel","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":279,"value":"### Création des isolignes\n_À l'aide de [d3-contour](https://github.com/d3/d3-contour)_\n\nPar défaut Graticule affiche 6 niveaux d'altitudes.","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":94,"value":"seuils = [0,500,1000,2000,3000,4000]","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":288,"value":"Initialisation du générateur de contour avec certains paramètres :\n+ `size`, la taille de la grille des données = largeur et hauteur de notre raster\n+ `thresholds`, chaque seuil génère une isoligne = nos 6 niveaux d'altitudes","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":92,"value":"contours = d3.contours().size([n, m]).thresholds(seuils)","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":293,"value":"**Génération des isolignes** \nPour chaque seuil, un geojson est crée.\n\n⚠️ À ce stade, les geojson ne sont pas du tout projetés. Les coordonnées correspondent au pixel du raster.","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":282,"value":"relief_XY = contours(values)","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":299,"value":"### Géoréférencement des isolignes\n_D'après une fonction inverse de Mike Bostock, https://observablehq.com/@d3/geotiff-contours-ii_\n\nLa fonction transforme les coordonnées des geojson en les passant de pixels à des coordonnées géographiques (lat/lon).","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":97,"value":"function invert(d) {\n const shared = {};\n\n let p = {\n type: \"Polygon\",\n coordinates: d3.merge(d.coordinates.map(polygon => {\n return polygon.map(ring => {\n return ring.map(point => {\n return [point[0] / n * 360 - 180, 90 - point[1] / m * 180];\n }).reverse();\n });\n }))\n };\n\n // Record the y-intersections with the antimeridian.\n p.coordinates.forEach(ring => {\n ring.forEach(p => {\n if (p[0] === -180) shared[p[1]] |= 1;\n else if (p[0] === 180) shared[p[1]] |= 2;\n });\n });\n\n // Offset any unshared antimeridian points to prevent their stitching.\n p.coordinates.forEach(ring => {\n ring.forEach(p => {\n if ((p[0] === -180 || p[0] === 180) && shared[p[1]] !== 3) {\n p[0] = p[0] === -180 ? -179.9995 : 179.9995;\n }\n });\n });\n\n p = d3.geoStitch(p);\n\n // If the MultiPolygon is empty, treat it as the Sphere.\n return p.coordinates.length\n ? {type: \"Polygon\", coordinates: p.coordinates}\n : {type: \"Sphere\"};\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":305,"value":"relief_LatLon = relief_XY.map(d => invert(d))","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":333,"value":"$32","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":370,"value":"viewof angle = Inputs.range([0, 360], {label: \"Direction de la lumière (angle)\", step: 5, value: 135})","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":104,"value":"$33","pinned":false,"hidden":false,"mode":"html","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":630,"value":"### Démonstration de l'indépendance de l'ombrage à la projection et ses paramètres","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":69,"value":"viewof basemapRes = Inputs.radio([\"110m\", \"50m\", \"10m\"], {value: \"110m\", label: \"Choisir la résolution du fonds de carte\"})","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":808,"value":"viewof projA = Inputs.radio(projs, {label: \"Choisir une projection\", value: projs.get(\"Equal Earth\")})","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":85,"value":"addMap(projA, true)","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":749,"value":"viewof rot = Scrubber(d3.range(0, 360+1,1), {autoplay: false, loop: true, alternate: false})","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":714,"value":"addMap(projB)","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":692,"value":"---\n## Paramètres\n### Générateur de carte","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":78,"value":"$34","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":81,"value":"projs = new Map ([\n [\"Atlantis\", d3.geoMollweide().rotate([30, -45, 90])],\n [\"Bertin 1953\", d3.geoBertin1953()],\n [\"Equal Earth\", d3.geoEqualEarth()],\n [\"LAEA\", d3.geoAzimuthalEqualArea().rotate([0,-90])],\n [\"Mollweide 2 hémisphères\", d3.geoInterruptedMollweideHemispheres()],\n [\"Orthographique\", d3.geoOrthographic().rotate([14,-40])],\n [\"Plate carrée\", d3.geoEquirectangular()],\n]) ","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":707,"value":"projB = d3.geoAzimuthalEqualArea().rotate([-90,-30, rot]).clipAngle(30)","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":158,"value":"land = topojson.merge(rawCountries, rawCountries.objects[`countries_${basemapRes}`].geometries)","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":574,"value":"### Déclaration du filtre svg d'ombrage de la carte","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":115,"value":"\n","pinned":true,"hidden":false,"mode":"html","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":433,"value":"### Calcul du décalage x,y de la démonstration du filtre svg et css\nLà où en représentation de relief on utilise la direction de la source de lumière, donc un angle, les filtres d'ombrage svg et css utilise un décalage x,y.\nLa fonction `getXYfromAngle()` permet de passer d'un angle en degré à un décalage x,y.","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":421,"value":"getXYfromAngle = (angle = 0, distance = 1.5) => {\n const degreeToRadian = (degree) => degree * Math.PI / 180\n const angleRadian = degreeToRadian(angle - 90)\n const x = distance * Math.cos(angleRadian)\n const y = distance * Math.sin(angleRadian)\n \n return [x.toFixed(3), y.toFixed(3)]\n}","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":689,"value":"Utilisation de la fonction","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":424,"value":"dxdy = getXYfromAngle(angle)","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":102,"value":"### Suivi du facteur de zoom\nPermet de garder des épaisseurs de tracé constante malgré le zoom. Idem aussi pour la distance de décalage de l'ombrage du relief.","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":510,"value":"mutable k = 1","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":39,"value":"---\n## Ressources et lib","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":41,"value":"","pinned":false,"hidden":false,"mode":"html","author":"$undefined","summary":"$undefined","output":"","database":"$undefined"},{"id":6,"value":"geotiff = require('geotiff')","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":100,"value":"d3 = require(\"d3\", \"d3-geo-projection\")","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":780,"value":"import {Scrubber} from \"@mbostock/scrubber\"","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":325,"value":"import {toc} from '@mbostock/toc'","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"}],"files":"$W35","stdlib":"1","titling":"auto"}} 30:"$2f:body"