Setting-up a New Map Layer – Part 5B: Simplifying Shapes in QGIS
Demonstrates how to simplify a shapefile using open-source QGIS software to create a GeoJSON. These GeoJSON files are used in Leaflet Maps in the IBIS-PH software and are necessary to display your data in a Leaflet map in IBIS.
Author: Chris Wenker, New Mexico Department of Health
Useful Resources:
https://en.wikipedia.org/wiki/Decimal...
https://gis.stackexchange.com/questio...
From the US Census’s metadata for TIGER/Line Shapefiles:
“Coordinates in the TIGER/Line Shapefiles have six decimal places, but the positional accuracy of these coordinates may not be as great as the six decimal places suggest.”
https://www2.census.gov/geo/pdfs/maps...
@3:45, re: Coordinate reference system and decimal degrees
A discussion of coordinate reference systems could form an entire instructional video in itself, but for the purposes of GeoJSON creation, here it was enough to determine if the source file was in a geographic coordinate system or a projected coordinate system.
The Census shapefile in this video was in a geographic coordinate system, which uses latitude/longitude values to reference positions. Latitude and longitude values are typically measured either in decimal degrees, or in degrees, minutes, and seconds. Because the QGIS Tolerance setting requests a decimal-based entry, using decimal degrees was appropriate here.
For a shapefile in a projected coordinate system, the Tolerance units to use would need to be determined by examining the projection system. E.g., a file in a UTM projection will use meters; a file in a State Plane system will likely use US feet.
https://gis.stackexchange.com/questio...
https://geonet.esri.com/thread/23160
https://en.wikipedia.org/wiki/Geograp...
http://desktop.arcgis.com/en/arcmap/1...
http://desktop.arcgis.com/en/arcmap/1...
Selecting a coordinate reference system during a GeoJSON export doesn’t seem to make much difference to the file’s performance in Leaflet, as long as it is a geographic coordinate system. Leaflet's default projection is EPSG: 3857, also known as "Google Mercator" or "Web Mercator."
https://gis.stackexchange.com/questio...
Another tool for vector simplification (untested by this author):
http://blog.mastermaps.com/2012/10/ma...
http://mapshaper.org/