Geospatial Memo

Recently I am learning a geospatial course at my home university, and I am also building a simple web application as exercise.

Following is an incomplete memo of what I have learned along the way.

• Leaflet.js is a good library for building map related applications. and you should check it out at https://github.com/Leaflet/Leaflet
• A basic code snippet of using leaflet.This creates a new map on DOM element whose id equals to “map” (which is usually a simple div), and add a basic Open Street Map base layer.

map = L.map('map').setView([1.3569, 103.7779], 12);

L.tileLayer('http://{s}.tile.osm.org/{z}/{x}/{y}.png', {
  attribution: '&copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors'
}).addTo(map);

• A few frequently used projection system

  EPSG:4326 – WGS 84
  EPSG:3857 – WGS84 Web Mercator (Auxiliary Sphere)
  EPSG:3414 – SVY21
  

  So what’s the difference between these projections?
  In short the data stored in Open Street Map is EPSG:4326, but The Open Street Map tiles and the WMS webservice are in the projected coordinate system that is based on the EPSG:3857.

  Read this more at http://gis.stackexchange.com/questions/48949/epsg-3857-or-4326-for-googlemaps-openstreetmap-and-leaflet

• When converting shp files into geojson, use EPSG:4326, which I found works well with leaflet. And I use GDAL to do so.

    ogr2ogr -f GeoJSON -t_srs EPSG:4326 [name].geojson [name].shp
  

  If you are Mac user, read more at http://ben.balter.com/2013/06/26/how-to-convert-shapefiles-to-geojson-for-use-on-github/

• Use Leaflet 0.8-dev version instead of 0.7 stable before 1.0 rolls out. In my opinion, the 0.8-dev version has more features and offer better utilities.