OSRExportToProj4 drops NAD83 datum in Proj4 string

[rsbivand]

A user of rgdal in R has noted that the WTK string in a file loses its Datum if the datum is NAD83, meaning that if the data is written out to file, the datum is not set. This can be illustrated by comparing the output of gdalinfo -proj4 for WKT and proj4 for input (written):

+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=23 +lon_0=-96 +x_0=0 +y_0=0 +datum=NAD83 +units=m +no_defs +ellps=GRS80 +towgs84=0,0,0

$ gdalinfo img1.tif ... PROJCS["unnamed",

GEOGCS["NAD83",

DATUM["North_American_Datum_1983",

SPHEROID["GRS 1980",6378137,298.2572221010002,

... PROJ.4 string is: '+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=23 +lon_0=-96 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs '

For two other cases, NAD27 and WGS84:

+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=23 +lon_0=-96 +x_0=0 +y_0=0 +datum=NAD27 +units=m +no_defs

$ gdalinfo -proj4 img2.tif ... PROJCS["unnamed",

GEOGCS["NAD27",

DATUM["North_American_Datum_1927",

SPHEROID["Clarke 1866",6378206.4,294.9786982139006,

... PROJ.4 string is: '+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=23 +lon_0=-96 +x_0=0 +y_0=0 +datum=NAD27 +units=m +no_defs '

and

+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=23 +lon_0=-96 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs

$ gdalinfo -proj4 img3.tif ... PROJCS["unnamed",

GEOGCS["WGS 84",

DATUM["WGS_1984",

SPHEROID["WGS 84",6378137,298.257223563,

... PROJ.4 string is: '+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=23 +lon_0=-96 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs '

There does not seem to be any reference to this in:

​http://www.gdal.org/ogr/classOGRSpatialReference.html#a28bd9b81856b6a93861576fff0933bb9

[rsbivand]

files for reported case