3

What's the difference between using CRS defined as

+proj=longlat +datum=WGS84 +no_defs

and a CRS defined as

+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs

Will using one or the other cause any problems. In the first case, how can GIS project properly without knowing which ellipsoid the data are referred to?

3

The datum=WGS84 includes the ellipsoid information hardcoded in PROJ:

http://svn.osgeo.org/metacrs/proj/trunk/proj/src/pj_datums.c

C_NAMESPACE_VAR struct PJ_DATUMS pj_datums[] = {
/* id       definition                               ellipse  comments */
/* --       ----------                               -------  -------- */
"WGS84",    "towgs84=0,0,0",                     "WGS84", "",

Apart from that, the WGS84 ellipsoid is default if no other is stated.

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