Best estimate for absolute global mean for 1951-1980 is 14.0 deg-C or 57.2 deg-F...
GHCN Processor 1.1 has a -abs option that causes the tool to write out "absolute measurement" averages, as opposed to temperature anomalies. Additionally, simulations I've run indicate that the tool's default cell and station combination method (the linear-equations-based method) is adequate for this sort of application.
You can get a global absolute measurement series with the following command.
ghchp -gt seg -abs -o /tmp/ghcn-global-abs.csv
In this case the tool says 4771 stations have been analyzed. GHCN v2 has considerably more stations, however. A lot of them are dropped in the adjusted data set because they don't have enough data points. To get a series based on the raw data, you can run:
ghcnp -dt mean -gt seg -abs -o /tmp/ghcn-global-abs-noadj.csv
The tool now analyzes 7067 out of 7280 stations. (GHCN Processor 1.1 has an implicit filter that drops any station without at least 10 data points in any given month.)
It probably shouldn't come as a surprise that there are differences in the results we obtain with each data set. With the adjusted data, the average for the baseline period 1950 to 1981 is 14.7°C. With the raw data, the average is 14.1°C. The reason for the discrepancy probably has to do with the sorts of stations that don't make it into the adjusted data set, typically because they haven't reported long enough. They might be cold stations, like stations near Antarctica, a region with a substantial scarcity of stations in the adjusted data set.
Let me post a graph of both series, just so there's no confusion.
So it's basically an offset difference between the two. If I'm correct about it being due to stations near Antarctica, one caveat would be elevation. It appears that stations in Antarctica are high up, and this is a problem. We could filter stations by elevation, but then we basically drop Antarctica, like the adjusted data set does.
I believe 14.1°C is probably a low estimate. It's also pretty clear that GHCN v2 is land-biased.
There does seem to be a slight slope difference (0.15°C/century) between the adjusted and raw series. This can't be anywhere near significance, and again, it probably has to do with the sorts of stations that don't make it into the adjusted data set. I wouldn't be surprised if "sceptics" manage to make a big deal of it, though.