The area I walk for exercise have lots of hills. I am going to take a wild guess and say the Sparks fitness tracker estimates a flat grade, therefore their calorie estimate will be significantly lower than actual for me.
I JUST received a Zephyr Heart Rate Monitor which links with the app Endomondo on my smart phone. My walk today was 6.12 km (3.8 miles) in 1 hour. The altitude change was 110 meters (360 feet) and the cumulative ascent (up and down and up and down etc) was 170 meters (557 feet).
Spark Fitness tracker said I burnt 348 calories.
My HR monitor said it was 642.
Either one is better than sitting on the sofa watching TV, but the HR monitor probably closer resembles my calories burnt.
Rather than presuming my HR monitor is a precise science instrument, I am using it to ensure my heart rate is at the correct intensity for an aerobic workout. Any workout becomes easier over time and I want to ensure that I continue to get a good aerobic workout.
It's my personal interval trainer in a box