Icepik: welcome to the Digital Forums!
Sarnia? not that far at all. You'll get the Detroit locals with a rooftop or attic antenna. if you aim yourself towards the Bluewater bridge/due west, you might even pick up the Flint locals, as well. Currently, no stations in southwestern ontario are operating on digital format, aside from CKXT-DT-3 on channel 19 in London.
The three sweet spots you'll want to aim for are SW towards the Southfield transmitter farm (where most of hte detroit locals, aside from WADL-TV/-DT broadcast from), SE towards Oil Springs (where most of the southwestern ontario locals broadcast from, and W towards Flint.
If you have a directional and rotor (or omnidirectional), your channel lineup should appear like so (using RF channels after june 12, only likely regularly-recievable channels shown):
WJBK-DT 7 (FOX) Detroit
CBET-TV 9 (CBC) Windsor
CFPL-TV 10 (A) London (if CFPL can reach Sanilac County, MI over-the-air, i'm sure you're within range of it)
WJRT-DT 12 (ABC) Flint
WKBD-DT 14 (The CW) Detroit
WSMH-DT 16 (FOX) Flint
CICO-TV-18 18 (TVOntario) London
WMYD-DT 21 (My Network TV) Detroit
WNEM-DT 22 (CBS) Flint
WFUM-DT 28 (PBS) Flint
CIII-TV-29 29 (Global) Oil Springs
WEYI-DT 30 (NBC, The CW/WBSF on DT2) Flint
WPXD-DT 31 (ION Television, Qubo on DT2, ION Life on DT3, Worship on DT4) Ann Arbor (not so sure about this one, but it's supposed to move to WKBD's tower and increase power to 1 megawatt, staying on 31...)
CBLN-TV-2 34 (CBC) Sarnia
WADL-DT 39 (Independent, The WORD Network on DT2) Mount Clemens, MI/Detroit, MI
WXYZ-DT 41 (ABC, Retro TV Net. on DT2, Weather Radar on DT3) Detroit
CKCO-TV-3 42 (CTV) Oil Springs
WTVS-DT 43 (PBS, PBS Kids on DT2, Create on DT3) Detroit
WWJ-DT 44 (CBS) Detroit
WDIV-DT 45 (NBC, This on DT2) Detroit
WBSF-DT 46 (The CW) Flint
W48AV 48 (ION Television/WPXD-DT1 simulcast) Warren, MI/Detroit
CHCH-TV-2 51 (E! Canada) Strathroy/London (if i can pick this up in windsor, you should have no problem doing so!)
CICO-TV-59 59 (TVOntario) Chatham
W66BV (TBN) Detroit, Approved to move to 47, broadcasts from the Ren Cen
CBLFT-TV-17 68 (CBC French) Oil Springs, Construction Permit to move to 17
CFMT-TV-1 69 (OMNI) London (This one has over a megawatt of power!)
Now, occasionally, you'll get e-skip across the lakes from stations in say, Toledo and Alpena, particularly during the summer (such as WTOL CBS 11 and WNWO NBC 24 in toledo) and WBKB (CBS 11 Alpena).
I'll give you the transition channels for the canadian signals if you want them.
If you want a basic indoor antenna, you'll be reduced to just the detroit signals (with possible dropouts) and the ones from Oil Springs, and MAYBE CFMT-TV-1 on 69. If you have an attic/rooftop antenna, you'll get most of these, and if you want to go all-out with a pre-amp, rotor and directional antenna, you'll get most to all of the stations on the list. Mind you, towers can get expensive, but you'll probably get everything within 120 miles if you have a 40-foot tower. if people in North London can pick up the detroit digital signals year-round (surprised their signals push that far to begin with, but north london's pretty high), i'm sure you can.
You can subtract some of the distance between you and the Detroit stations because of Lake St. Clair. Water is a great reflector of signals, which is why folks in London watch Erie, PA's locals, or how i pick up Toledo's analog signals here in downtown Windsor.