Add citation info to README (#87)

resolves #86

Author: pdil

NEWS.md

@@ -1,5 +1,6 @@
 # [unreleased]
 
+* Add citation information to README, see [Issue #86](https://github.com/pdil/usmap/issues/86).
 
 # usmap 0.7.0
 Released Saturday, January 20, 2024.

README.Rmd

@@ -249,6 +249,17 @@ data %>% rowwise %>% mutate(fips = fips(state, county))
 
 ## Additional Information
 
+### Citation Information
+
+The images generated by `usmap` are not under any copyright restrictions and may be used and distributed freely in any publication or otherwise. 
+
+The underlying shapefiles used to generate the map data are derived from the [US Census Bureau's TIGER/Line Shapefiles](https://www.census.gov/geographies/mapping-files/time-series/geo/tiger-line-file.2022.html#list-tab-790442341) which are not copyrighted but do suggest citation. See [section 1.2 of this document](https://www2.census.gov/geo/pdfs/maps-data/data/tiger/tgrshp2022/TGRSHP2022_TechDoc_Ch1.pdf).
+
+If you wish to cite `usmap` in a publication (appreciated but never required!), you may do so in the following way:
+```{r citation}
+citation("usmap")
+```
+
 ### Coordinate System
 
 `usmap` uses the [US National Atlas Equal Area](https://epsg.io/9311) coordinate system:

README.md

@@ -250,6 +250,39 @@ data %>% rowwise %>% mutate(fips = fips(state, county))
 
 ## Additional Information
 
+### Citation Information
+
+The images generated by `usmap` are not under any copyright restrictions
+and may be used and distributed freely in any publication or otherwise.
+
+The underlying shapefiles used to generate the map data are derived from
+the [US Census Bureau’s TIGER/Line
+Shapefiles](https://www.census.gov/geographies/mapping-files/time-series/geo/tiger-line-file.2022.html#list-tab-790442341)
+which are not copyrighted but do suggest citation. See [section 1.2 of
+this
+document](https://www2.census.gov/geo/pdfs/maps-data/data/tiger/tgrshp2022/TGRSHP2022_TechDoc_Ch1.pdf).
+
+If you wish to cite `usmap` in a publication (appreciated but never
+required!), you may do so in the following way:
+
+``` r
+citation("usmap")
+#> To cite package 'usmap' in publications use:
+#> 
+#>   Di Lorenzo P (2024). _usmap: US Maps Including Alaska and Hawaii_. R
+#>   package version 0.7.0, <https://CRAN.R-project.org/package=usmap>.
+#> 
+#> A BibTeX entry for LaTeX users is
+#> 
+#>   @Manual{,
+#>     title = {usmap: US Maps Including Alaska and Hawaii},
+#>     author = {Paolo {Di Lorenzo}},
+#>     year = {2024},
+#>     note = {R package version 0.7.0},
+#>     url = {https://CRAN.R-project.org/package=usmap},
+#>   }
+```
+
 ### Coordinate System
 
 `usmap` uses the [US National Atlas Equal Area](https://epsg.io/9311)

vignettes/usmap3.html

@@ -12,7 +12,7 @@
 
 <meta name="author" content="Paolo Di Lorenzo" />
 
-<meta name="date" content="2024-01-19" />
+<meta name="date" content="2024-01-23" />
 
 <title>3. Advanced Mapping</title>
 
@@ -340,12 +340,12 @@
 
 <h1 class="title toc-ignore">3. Advanced Mapping</h1>
 <h4 class="author">Paolo Di Lorenzo</h4>
-<h4 class="date">2024-01-19</h4>
+<h4 class="date">2024-01-23</h4>
 
 
 
 <p>This vignette will explore some of the more advanced mapping features
-of <code>usmap</code>. Before continuing, be sure to check out <a href="mapping.html">Mapping the US</a> as that will cover more of the
+of <code>usmap</code>. Before continuing, be sure to check out <a href="usmap2.html">Mapping the US</a> as that will cover more of the
 basics of plotting US maps and styling them with
 <code>ggplot2</code>.</p>
 <div id="labels" class="section level2">