Yeah, I use a Garmin watch with HRM. Kinda pricey, but like said, the only way to accurately overlay a GPS track with HR. Also, mine is good for about 7 hours of tracking. Found that out on a MTB ride this past summer I should not have been on
The first time I used mine on the bike, I was amazed that after about an hour of riding my average HR was 143. Sounds good, but based on my age it should have been in the mid-150s. Still a good workout, but the slightly higher heart rate is better at burning calories. Too high and you start burning protein, so that is why you want to monitor and keep it in a range.