CODEWITHSUNDEEP
rvectormappinggisraster
MCNC
MCNC

Reputation: 69

Rasterize error: Polygon to raster conversion produces horizontal lines

I am working with shapefiles in R that I need to convert from polygon to raster. While the vectors look perfect when plotted, when converted to raster using 'rasterize' they produce erroneous horizontal lines. Here is an example of the problem:

Polygon Raster

Here is a generic example of the code that I am using (sorry that I cannot upload the data itself as it is proprietary):

spdf.dat <- readOGR("directory here", "layer here")

# Plot polygon
plot(spdf.dat, col = 'dimgrey', border = 'black')

# Extract boundaries
ext <- extent(spdf.dat)

# Set resolution for rasterization
res <- 1 

# determine no. of columns from extents and resolution
yrow <- round((ext@ymax - ext@ymin) / res)
xcol  <- round((ext@xmax - ext@xmin) / res)

# Rasterize base
rast.base <- raster(ext, yrow, xcol, crs = projection(spdf.dat))

# Rasterize substrate polygons
rast <- rasterize(spdf.dat, rast.base, field = 1, fun = 'min', progress='text')
plot(rast, col = 'dimgrey')

Does this seem to be a problem with the source data or the rasterize function? Has anyone seen this sort of error before? Thank you for any advice that you can provide.

Upvotes: 1

Views: 1484

Answers (3)

mdwhitby
mdwhitby

Reputation: 41

Just as a follow up to this question based on my experiences.

The horizontal lines are as a result of these 'islands' as described above. However, it only occurs if the polygon is 'multi-part'. If 'islands' are distinct polygons rather than a separate part of one polygon, then raster:rasterize() works fine.

Upvotes: 1

Craig Mohn
Craig Mohn

Reputation: 11

I had a similar problem rasterizing the TIGER areal water data for the San Juan Islands in Washington State , as well as for Maui - both of these spatial polygon data frames at the default resolution returned by package Tigris using a raster defined by points 1 arc-second of lat/lon apart. There were several horizontal stripes starting at what appeared to be sharp bends of the coastline. Various simplification algorithms helped, but not predictably, and not perfectly.

Try package Velox, which takes some getting used to as it uses Reference Classes. It probably has size limits, as it uses the Boost geometry libraries and works in memory. You don't need to understand it all, I don't. It is fast compared to raster::rasterize (especially for large and complicated spatial lines dataframes), although I didn't experience the hundred-fold speedups claimed, I am not gonna complain about a mere factor of 10 or 20 speedup. Most importantly, velox$rasterize() doesn't leave streaks for the locations I found where raster::rasterize did!

I found that it leaves a lot of memory garbage, and when converting large rasterLayers derived from velox$rasterize, running gc() was helpful before writing the raster in native R .grd format (in INT1S format to save disk space).

Upvotes: 1

Bastien
Bastien

Reputation: 3178

To make it official so the question is considered answered, I'll copy my commented responses here. You can therefor accept it.

When I look at your figure, it seems to me that the problematic appearing lines in the raster are situated at the same latitude of some islands. Try to removes these islands from your dataset. If the problem disappear, you'll know that your data is the problem and where in your data the problem lies.

An other option is to try the gdalUtils package which has a function: gdal_rasterize. Maybe gdal is less exigent in the input data.

Upvotes: 1

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