The tool shows a Google map with the usual facilities, but with markers for the stations in GHCN V4. You can click on a marker to bring up some details. The main utility is that you can choose different marker colors for different subsets. An important color is "Invis". It earlier stood for invisible, though it is now implemented by removing the marker from the map (you can get it back).
At the right you can see an orange-background table of the color options, and a cyan table of selection criteria. The selections come with a more/less sign that you can toggle, and a textbox where you can write a criterion number. Tick the left checkbox to make it live. The color table has radio buttons to make the color happen, and on the other side a count of how many of that color there are.
Nothing happens until you then click on a color radio button. When you do, the markers are sorted according to the live (checked) selection options, and those that qualify change to that color. This is "and" logic; all criteria have to be satisfied. You'll find it useful to sometimes switch to negative logic - make Invisible the options you don't want. If no checkboxes are ticked, everything will change.
I've included Lat and Lon so that you can look at a restricted area. I was thinking of performance there - the map can get sluggish if too many markers are visible. In fact, I haven't had performance problems, but that may be because my computer has upgraded.
At the bottom a selection box allows you to make color itself an option. You can subset from the classes you have discriminated.
Some examples of things you might want to do:
- Set endyr>2014 to pink the stations currently reporting. Or set endyr<2015 and then Invis to remove those that are not
- Begin by making everything Invis (just click it).
- Set duration >100 to pink (or cyan etc) long duration stations.
- Set startyr>1850 and Invis to leave only the very early stations
The pop-up tags give dates, name and the GHCN Inventory number. The first two letters of that are an abbreviation for the country (details here).
Some altitudes are missing. I have set them to -999, and the default criterion to -1000, so you can see them all.