See article.
Kentucky maintains a historical marker database(KHMD). This project is in two discrete parts: the building of a dataframe for the map markers and the display of the markers on a leaflet map.
The KHMD fields are not assigned separate html containers so the effort transformed from web scraping (rvest) to a regex (stringr) exercise. Or, in other words, it was a jumbled mess. However, the information is both extensive and informative. The primary constraint for the mapping of Kentucky’s historic markers was the lack of coordinates.
Scraping the database yielded 2226 total markers. The markers are numbered between 1 and 2,610. There are 412 that are not included in the database for whatever reason, including poor execution on my part. The database did have the county where the markers are located so the center of each county was computed and the markers were assigned there.
For most counties that worked pretty well. For example, the center of Jefferson County is Louisville and locating the markers there worked well. However, they were stacked on top of each other. Jefferson County has 228 markers.
With 228 markers assigned the same coordinates, the "pop-up" contents were not easily viewed. A minimal amout of random noise was introduced ("jitter") so the pop-up contents could be displayed. This ended up with markers placed on the top of strip malls and car dealerships. The map tiles were changed to Stamen so that building outlines were excluded.
The header code must reference jquery, leaflet and leaflet market-cluster. Leaflet, mentioned above, has excellent documentation. Crucial for the markers to cluster on the Stamen map are the following lines of code added to the header of the webpage:
<!--stylesheets load first-->
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/leaflet/1.6.0/leaflet.css" />
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/leaflet.markercluster/1.4.1/MarkerCluster.css" />
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/leaflet.markercluster/1.4.1/MarkerCluster.Default.css" />
<!--scripts load next-->
<script type='text/javascript' src='https://cdnjs.cloudflare.com/ajax/libs/jquery/3.4.1/jquery.min.js'></script>
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/leaflet/1.6.0/leaflet.js"></script>
<script type='text/javascript' src='https://cdnjs.cloudflare.com/ajax/libs/leaflet.markercluster/1.4.1/leaflet.markercluster.js'></script>
<script type='text/javascript' src="https://cdnjs.cloudflare.com/ajax/libs/leaflet-providers/1.9.1/leaflet-providers.min.js"></script>
Kentucky counties were added as a layer to the map from the Kentucky Open GIS portal. An API for the full geojson feature collection is available here.
#install.packages("devtools")
devtools::install_github("RobWiederstein/my_functions")
#customize
git init
git clone https://github.com/RobWiederstein/my_functions.git
Distributed under the MIT License. See LICENSE for more information.
Rob Wiederstein @RobWiederstein - khuon68@gmail.com
Project Link: https://github.com/RobWiederstein/covid-19-ky