SVG can be drawn on map in much more faster way than traditional approaches, at least for points. Traditional approach re-position each element to fit into the view of the map, however SVG is “scalable” so we can use it and it performs much more faster for zoom-in/out.
SVG itself define viewport by its coordinate space, all outside of this viewport is usually clipped, so it is important to keep SVG viewport in-line with the viewport of the map. There are approaches that resizes SVG as you zoom-in (here), and while it works, it has a problems in deep-zooms when you need to move on map (actually you move giant SVG based on the zoom )
translating LatLon to absolute pixel values (like here used for WebGL) is possible solution, however IE and FF has problems with large numbers for transoform (>1 M), So we need to get SVG elements in view coordinates and translate them.
Having some track of bounding box of all elements like again used here should be avoided (SVG or its group element knows about extension of the elements it holds)
So while we keep SVG in the viewport, we need to compensate any shift and zoom by translating <g> (group) of all elements.
So in leaflet when map moves, SVG is translated back to its original position while <g> is translated forward to reflect the map movement
We need to keep track of LatLon position of either map center or one of the corner – we use topLeft corner.
Leaflet doesn’t do precise enlargement and rounds view points because of some CSS troubles on some devices (noted here). We need to patch two translating functions in Leaflet to get this right (so SVG enlargement will be aligned with map)… but I need to look on this again, best would be to not patch Leaflet of course.
most important things happen in moveEnd event:
var bounds = this._map.getBounds(); // -- latLng bounds of map viewport
var topLeftLatLng = new L.LatLng(bounds.getNorth(), bounds.getWest()); // -- topLeft corner of the viewport
var topLeftLayerPoint = this._map.latLngToLayerPoint(topLeftLatLng); // -- translating to view coord
var lastLeftLayerPoint = this._map.latLngToLayerPoint(this._lastTopLeftlatLng);
var zoom = this._map.getZoom();
var scaleDelta = this._map.getZoomScale(zoom, this._lastZoom); // -- amount of scale from previous state e.g. 0.5 or 2
var scaleDiff = this.getScaleDiff(zoom); // -- diff of how far we are from initial scale
this._lastZoom = zoom; // -- we need to keep track of last zoom
var delta = lastLeftLayerPoint.subtract(topLeftLayerPoint); // -- get incremental delta in view coord
this._lastTopLeftlatLng = topLeftLatLng; // -- we need to keep track of last top left corner, with this we do not need to track center of enlargement
L.DomUtil.setPosition(this._svg, topLeftLayerPoint); // -- reset svg to keep it inside map viewport
this._shift._multiplyBy(scaleDelta)._add(delta); // -- compute new relative shift from initial position
// -- set group element to compensate for svg translation, and scale</pre>
this._g.setAttribute("transform", "translate(" + this._shift.x + "," + this._shift.y + ") scale(" + scaleDiff + ")");
Recent months I have been programming “Green Space Analyzer” web app that shows modern approach to visualize and query multi temporal geospatial data. User see information in a form he can interact with and discover new patterns, phenomena or information just by very fast ‘feed-back’ of the UI response on the user input. When user selects for example certain area, all graphs instantly animates transition to reflect selection made, this helps to better understand dynamics of the change. Animation can be seen everywhere – from labels on bar chart, through colors change of the choropleth up to title summary. it creates subtle feeling of control or knowing what has changed and how it has changed. At HxGN 15 conference in hexagon geospatial keynote, CEO Mladen Stojic showcased it as part of the vision called Smart M.App, worth to look at (at 52:40 starts Smart M.App demo):
For this year HxGN14 conference I have prepared a web app of modern data vizualisation, I have got inspired by great ideas from Victor Bret and his research and talks for general concept (high interactivity, visualization ) of this app.
It is exciting to see what is possible to do today inside browser and interactivity provided by various open source projects (e.g. leaflet,d3 and its plugins) and WebGL technology .
D3 can plot map in various projections, however do not expect to get same set of (overlapping) functionality as Leaflet or OpenLayers. D3 can extend these mapping frameworks. For OL3 simple example is here for Leaflet, my favourite is hexbins or check my own experiment here.
While D3 is kind of ‘base’ charting library (lot of utility functions, helpers) , there is upper , high level library too that can provide lot of ‘boilerplate’ for common charts. Here comes great news:
Update August 2015: check “scaled”- based fast SVG rendering on top of Leaflet here
Testing SVG limits of plotting points on map using D3, Leaflet following this base sample:http://bost.ocks.org/mike/leaflet. However instead of scaling SVG in deep zooms, I am using Enter/Update/Exit pattern from D3 to dynamically update points on map. This has been prototyped also here http://bl.ocks.org/sumbera/9972460 with brushing of 100T points.
For zooming out (causing all points to be displayed), I am filtering out points that can’t be effectively visible, thus reducing number of points in SVG. (check console for log output).
This sample is using real data of 24T coordinates where points are clustered around cities, rather than artifically randomized. Real number of rendered points / removed points can be seen in console
“Well, SVG doesn’t scale well to large numbers of objects, but Canvas doesn’t scale well to large screens”
“SVG performance degrades quickly (exponentially on Safari?) in the number of objects, but Canvas performance remains at a near-constant low. This makes sense, since Canvas is just a bitmap buffer, while SVG has to maintain additional references to each object that it renders. Also, though not pictured, note that performance in clearing an SVG element also decreases in the number of drawn objects.”