I am installing a large pergola towards the back of my garden, that goes the full width of the garden (roughly around 1065cm in width total).
I am looking at installing a post in each of the four corners firstly and then add in further posts at varying intervals to perform the structure. My issue I am not sure how to get around, is ensuring that the heights are completely level. The garden isn't completely straight and obviously digging the holes around 60cm deep, with 10cm filled with gravel and the other 50cm for the post below ground. I have dug the holes as close to 60cm as possible all around, but it's never going to be perfect - so not sure how to ensure all heights are the same heights - any advice?
If the posts were closer, I would get a board and add to the top and use a spirit level - but as they are stretched apart a lot - I can't do that?
The alternative would be to start adding in the middle posts first where I can use the board and spirit level approach, however I was hoping to do the corners first, to ensure all is completely squared etc.
What would you suggest?