CODEWITHSUNDEEP
rgismap-projections
Leah Wasser
Leah Wasser

Reputation: 717

project from Geographic WGS84 to Mercator in R - points not finite

I am running into some projection issues in R that I do not understand.

I have downloaded the following global dataset:

http://www.naturalearthdata.com/downloads/110m-physical-vectors/110m-land/

I then am creating maps using different projections to teach the concept of spatial projection.

I have successfully mapped and reprojected / mapped the data in lat/lon WGS84 and reprojected to Robinson 

library(rgdal)
library(ggplot2)
setwd("~/Documents/data")

# read shapefile
worldBound <- readOGR(dsn="Global/Boundaries/ne_110m_land", 
                layer="ne_110m_land")

# convert to dataframe
worldBound_df <- fortify(worldBound)  

# plot map
ggplot(worldBound_df, aes(long,lat, group=group)) +
  geom_polygon() +
  labs(title="World map (longlat)") +
  coord_equal() +
  ggtitle("Geographic - WGS84 Datum")

# reproject from longlat to robinson
worldBound_robin <- spTransform(worldBound,
                          CRS("+proj=robin"))
worldBound_df_robin <- fortify(wmap_robin)
ggplot(worldBound_df_robin, aes(long,lat, group=group)) +
  geom_polygon() +
  labs(title="World map (robinson)") +
  coord_equal()

Now - when i try to project to Mercator WGS84 i run into issues.

# reproject from longlat to mercator
worldBound_merc <- spTransform(worldBound,
                          CRS("+init=epsg:3395"))
# make ggplot happy
worldBound_df_merc <- fortify(worldBound_merc)
# plot map
ggplot(worldBound_df_merc, aes(long,lat, group=group)) +
  geom_polygon() +
  labs(title="World map (Mercator WGS84)") +
  coord_equal()

I get the error: Error in .spTransform_Polygon(input[[i]], to_args = to_args, from_args = from_args, : failure in Polygons 8 Polygon 1 points In addition: Warning message: In .spTransform_Polygon(input[[i]], to_args = to_args, from_args = from_args, : 2 projected point(s) not finite

The error is on the spTransform function. It almost seems as if it can not calculate a finite xy coordinate from lat long to mercator for some points but i don't understand how to fix /address this. My searches on this site and others have resulted in other instances of this error, but not a good explanation of what triggers the error when projecting the data so i can fix it.

Thank you for any guidance! Leah

Resolution Code: For those who run into this - i just cropped the data to be able to plot using Mercator. This is just a demonstration so it's ok to lose some of the data for visual, mapping purposes.

# create extent object from world data
newExt <- extent(worldBound)
# redefine the extent to the limits of mercator EPSG 3395
newExt@ymin <- -80
newExt@ymax <- 80

# crop data to new extent
merc_WorldBound <- crop(worldBound,
                        newExt)

# reproject from longlat to mercator
worldBound_merc <- spTransform(merc_WorldBound,
                          CRS("+init=epsg:3395"))

Upvotes: 2

Views: 1899

Answers (1)

user5219763
user5219763

Reputation: 1294

The issue appears to be points that lie outside the bounds of the epsg:3395 projection (-180, -80, 180, 84) http://spatialreference.org/ref/epsg/wgs-84-world-mercator/. To correct this you can clip the shapefile to an appropriate extent and then perform the reprojection.

library(raster)
library(rgdal)
worldBoundClipped <- crop(worldBound,extent(-180,180,-84,80))
worldBound_merc <- spTransform(worldBoundClipped,CRS("+init=epsg:3395"))

Upvotes: 4

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