Installation

Currently you will find projView only in the development branch of mapview. If you want to use it the probably easiest way to install mapview is to use library("devtools"). With devtools installed and loaded type the following:

For the development version:

install_github("environmentalinformatics-marburg/mapview", ref = "develop")

Some remarks about projected slippy maps

Why would you be interested to visualize projected data? The answer is two folded but simple. First there are areas of the earth which you cannot map with a pseudo-Mercator projection and second (at least for me) even more important the adequate or inadequate cartographic representation of spatial data has a deep impact on our visual reception and interpretation of this data. The straightforward conclusion is that there is a high need for projected mapping.

Beyond all mapping philosophy let’s focus on a practical problem. We want to analyze and visualize some recent research data from the antartic continent using R and slippy maps. There is no possibility to use leaflet maps beyond the -85 degree latitude due to the implemented projections and we have to switch over to the great Open Source Geographic Information System QGIS or another mature GIS software package. With no doubt we may also use the well known static plotting packages of R - but no slippy maps for the penguins and polar bears within R.

Leaflet is a fascinating JavaScript library which is not only lightweight and easy to use. but moreover fully extensible with plug-ins and own JS scripts. With respect to our needs the proj4leaflet extension of leaflet is dealing with projections. The R interface to JS-leaflet is provided by the leaflet package and in turn the mapview package provides a comfortable wrapper for the R leaflet package. It seems to be obvious that this chaining may lead to some restrictions and exactly this is the situation.

ProjView uses its own interface directly to leaflet. To keep it in line with leaflet and mapview it is also using the htmlwidget package. the main advantage is that there is no need to live with the restrictions of the outlined chain but on the other hand you have to to it by your own means. Therefore projView is not neat less integrated and its usage and functionality is slightly different and restricted.

Use case EPSG:3031

As we already have learned, the mapview penguin story ends at 85 degree of latitude. Using projView continuous the story. Unfortunately first we need to invest in the business - we have to gather some informations.

The short way

You know what to do? Perfect. Search “NASA leaflet earthdata” and choose the appropriate GIBS Map Library Usage hit. Navigate to Leaflet and choose GIS Web examples. Find Antarctic (EPSG:3031) and move down in this tutorial to parameter format for use in preview.

The long way

<Layer>
<ows:Title>BlueMarble_ShadedRelief_Bathymetry</ows:Title>
<ows:LowerCorner>-180 -90</ows:LowerCorner>
<ows:UpperCorner>180 -38.941373</ows:UpperCorner>
</ows:WGS84BoundingBox>
<ows:BoundingBox crs="urn:ogc:def:crs:EPSG::3031">
<ows:LowerCorner>-4194304 -4194304</ows:LowerCorner>
<ows:UpperCorner>4194304 4194304</ows:UpperCorner>
</ows:BoundingBox>
<ows:Identifier>BlueMarble_ShadedRelief_Bathymetry</ows:Identifier>
<Style isDefault="true">
<ows:Title>default</ows:Title>
<ows:Identifier>default</ows:Identifier>
</Style>
<Format>image/jpeg</Format>
<TileMatrixSetLink>
<TileMatrixSet>EPSG3031_500m</TileMatrixSet>
</TileMatrixSetLink>
<ResourceURL format="image/jpeg" resourceType="tile" template="http://map1.vis.earthdata.nasa.gov/wmts-antarctic/BlueMarble_ShadedRelief_Bathymetry/default/{TileMatrixSet}/{TileMatrix}/{TileRow}/{TileCol}.jpg"/>
</Layer>

map.types parameter list of projView

Now we need an appropriate structure for these parameters to make it less complicated to feed the projView function with this information. This is a simple nested list. If we fill it with the values it looks like this:

NASA=list(service="OSM",                                             # name of the list
L.tileLayer="https://map1{s}.vis.earthdata.nasa.gov/wmts-antarctic/", # base address <ResourceURL>
# layer list is structured like the path of the url in this case: <ows:Identifier>,<TileMatrixSet>
          layer=list( "BlueMarble_ShadedRelief_Bathymetry"   = list("BlueMarble_ShadedRelief_Bathymetry","default","EPSG3031_500m","{z}/{y}/{x}"),
                      "AMSR2_Sea_Ice_Brightness_Temp_6km_89H"= list("AMSR2_Sea_Ice_Brightness_Temp_6km_89H","default","2014-02-04","EPSG3031_1km","{z}/{y}/{x}"),
                      "MODIS_Terra_Snow_Cover"               = list("MODIS_Terra_Snow_Cover","default","2014-02-04","EPSG3031_1km","{z}/{y}/{x}")
          ),      # end of layer list
 
format="image/jpg",#<Format>
tileSize="512",   # min resolution  EPSG:3031 examples
subdomains="abc", # most often used -> can be assumed as default 
noWrap ="true",   # most often used -> can be assumed as default 
# attribution has to meet the data providers needs and your personal interpretation of transparence 
attribution="<a href='https://wiki.earthdata.nasa.gov/display/GIBS'> NASA EOSDIS GIBS</a> &nbsp;|| &nbsp; <a href='https://github.com/kartena/Proj4Leaflet'> Proj4Leaflet</a> | Projection: <a href='http://spatialreference.org/ref/epsg/wgs-84-antarctic-polar-stereographic/'> EPSG3031</a>",
params=list(t_epsg="EPSG:3031", # exactly this the EPSG Code beside all other possibilieties and standards if possible keep it like this
t_srs="+proj=stere +lat_0=-90 +lat_ts=-71 +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs", # corresponding proj4 strings best be taken from href='http://spatialreference.org
mapCenter=list(cLat="-90",  #see example map EPSG:3031 examples
cLon="0"),                  #see example map EPSG:3031 examples
initialZoom="1",            #EPSG:3031 examples: Please note that the "top tile" (i.e., zoom level 0) is not currently supported by GIBS.
zoomLevels="5",             # zoom/levels EPSG:3031 examples
initialResolution="256",           # = minresolution EPSG:3031 examples
ovlBounds=list(minx="-4194304", ## <ows:LowerCorner>
miny="-4194304",                ## <ows:LowerCorner>
maxx="4194304",                 ## <ows:UpperCorner>
maxy="4194304"),                ## <ows:UpperCorner>
origin=list(olx="-4194304",    ## <ows:LowerCorner> firstvalue minx
oly="4194304"),                ## <ows:UpperCorner> lastvalue maxy
relUrl=""                      ## obsolete used for local tiles
))

List of 11
 $ service    : chr "OSM"
 $ L.tileLayer: chr "https://map1{s}.vis.earthdata.nasa.gov/wmts-antarctic/"
 $ layer      :List of 3
  ..$ BlueMarble_ShadedRelief_Bathymetry   :List of 4
  .. ..$ : chr "BlueMarble_ShadedRelief_Bathymetry"
  .. ..$ : chr "default"
  .. ..$ : chr "EPSG3031_500m"
  .. ..$ : chr "{z}/{y}/{x}"
  ..$ AMSR2_Sea_Ice_Brightness_Temp_6km_89H:List of 5
  .. ..$ : chr "AMSR2_Sea_Ice_Brightness_Temp_6km_89H"
  .. ..$ : chr "default"
  .. ..$ : chr "2014-02-04"
  .. ..$ : chr "EPSG3031_1km"
  .. ..$ : chr "{z}/{y}/{x}"
  ..$ MODIS_Terra_Snow_Cover               :List of 5
  .. ..$ : chr "MODIS_Terra_Snow_Cover"
  .. ..$ : chr "default"
  .. ..$ : chr "2014-02-04"
  .. ..$ : chr "EPSG3031_1km"
  .. ..$ : chr "{z}/{y}/{x}"
 $ format     : chr "image/jpg"
 $ tileSize   : chr "512"
 $ subdomains : chr "abc"
 $ minZoom    : num 0
 $ maxZoom    : num 5
 $ noWrap     : chr "true"
 $ attribution: chr "<a href='https://wiki.ear __truncated__'"
 $ params     :List of 9
  ..$ t_epsg           : chr "EPSG:3031"
  ..$ t_srs            : chr "+proj=stere +lat_0=-90 +lat_ts=-71 +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs"
  ..$ mapCenter        :List of 2
  .. ..$ cLat: chr "-90"
  .. ..$ cLon: chr "0"
  ..$ initialZoom      : chr "0"
  ..$ zoomLevels       : chr "5"
  ..$ initialResolution: chr "256"
  ..$ ovlBounds        :List of 4
  .. ..$ minx: chr "-4194304"
  .. ..$ miny: chr "-4194304"
  .. ..$ maxx: chr "4194304"
  .. ..$ maxy: chr "4194304"
  ..$ origin           :List of 2
  .. ..$ olx: chr "-4194304"
  .. ..$ oly: chr "4194304"
  ..$ relUrl           : chr ""

        maxResolution <- res / div
        resolution<- list()
        for ( i in seq(0,maxZoom)){
        resolution[i+1] <- maxResolution /  2^i
        }

       initialResolution=c(8192,4096,2048, 1024, 512, 256, 128,64, 32, 16, 8, 4, 2,1,0.5,0.25,0.125,0.0625)

    minx=NULL
    

Some examples

library(mapview)
library(raster)

 # load data of the arctic stations
 data("campsQ2")

 # load the list of the above example
 data("map.types")
 

 
mapview::projView(campsQ2,  map.types = map.types$NASA)

  # load data of the arctic stations
 data("campsQ2")

 # load the list of the above example
 data("map.types")
 
 nasa<-map.types$NASA
 
### use the visEarthPole function as a plugin
projView(campsQ2, map.types = nasa,
         internalList = TRUE,
         externalList = c("arctic-nasa","visEarthPole(groupList='1000',dateString='2014-02-04',createList = TRUE)"))

Final remarks

There are still more limitations than solutions. Actually the process of generating the correct map.types lists is more ore less cumbersome. On the other hand in the end there is always some manual work due to non strict service declarations. If somebody likes the function it could be worthwhile to automated this process.

In future releases I would like to

• provide some additional functions for often used datasets
• integrate a fully working localTile functionality for serving local raster data
• …

If you have any feedback, please don’t hesitate to contact me.

Bug reports should be filed at https://github.com/environmentalinformatics-marburg/mapview/issues

Cheers

Chris