I just did a wee video about methods of estimating head and put it on Youtube. It’s a bit rough in places, but I hope it helps.
Shown in this video are the following methods:
- Using a contour map (in this case an ordnance survey map taken on streetmap.co.uk)
- Using Google Earth Pro to make a track with an elevation profile
- Using a Johnson Sight Level that you can buy on eBay or wherever
- Using a “clinometer” app on your phone
- Using a pressure gauge on a pipe full of water
I did not mention that you can also use the clinometer to measure the angle of slope, and a long tape to measure distances, and thereby you can calculate the drop based on the triangle calculation for each step of the way. This can also work well if the device is calibrated. It also gives you an accurate pipe length.
An accurately calibrated aneroid altimeter can be a possible method on high head sites, but be wary of using GPS altimeters as they are not likely to be very accurate. Nor is Google Earth, to be honest. Each of these methods just gives you another rough indication which is why you should use several of them if possible and see if they agree at all with each other. There are more methods for sure including laser levels and tubes full of water but these are ones that I actually use.
In the end you just need this head measurement to satisfy yourself that your hydro site is going to delivery useful energy. As a ballpark figure, the output in Watts might be about 5 Watts for every metre of head and litre/second of flow. So if you have 100m head and 2 l/s flow then you can likely make 5 x 100 x 2 = 1000W or 1kW of power, which after 24 hours adds up to 24kWh units of electrical energy per day. More than an average home uses.