Just wanted to touch on the differences between a few things.
First off there is NO such thing as a "30 day tourist visa", neither is there a "30 day visa on arrival" :O The simple way to look at it is if you didn't BUY a tourist visa from a thai consulate before you winged your way here you do NOT have a tourist visa and if you didn't pay for a visa when you arrived here you don't have a visa on arrival.
If you showed up here without having bought a visa from a thai consulate beforehand and were stamped in for free (usually for 30 days but that's country dependent) you came in on whats known as a visa exempt entry.
Visa exempt entries are available to people from 57 countries. It breaks down like this; 5 countries get 90 day visa exempt entries, 6 countries get 30 day "visa waivers", 44 countries get 30 day visa exempt entries & 2 countries get 14 day visa exempt entries.
People who enter on 30 day visa exempt entries can apply for a 30 day extension at the immigration office for 1900baht.
People who enter on 90 day visa exempt entries (5 countries), people who come in on 30 day visa waiver (6 countries) * people who come in on 14 day visa exempt entries cannot get a 30 day extension.
Visas on arrival are available to people from 21 countries.
They BUY a 15 day visa on arrival for 2000baht when they get here.
So to recap one last time; IF you didn't buy a visa from a consulate before you came and didn't buy a visa from the immigration people when you stamped in you're NOT here on any type of visa, you're here on a visa exempt entry ;)
Also no matter what clueless websites on the inter-web state there are no more 15 day visa exempt entries by land for anyone :/ They were discontinued back on Dec 31st 2016. That's when they went back to 30 day visa exempt entries by land BUT put a limit of two by land in a calendar year (Jan 1 - Dec 31) for people from 44 countries.
When talking about visas/extensions etc it is very important to get the terminology correct. As you might surmise I am quite a stickler for correct terminology.
Sorry this was long, hopefully it is semi-useful