thanks Ramblinrev for the sewing tips -- very helpful... was considering grosgrain on the edges, now i'm thinking of not... will definitely be testing some practice swatches beforehand.
Last night i figured out how I'm going to cut/measure my cats (whichever ones I decide to do). I'm at work, so I can't explain here, but I'll be taking pictures and if it works (and maybe even if it doesn't) I'll post up my cat-cutting procedure from the perspective of someone who has never done this before.
Thanks again to everyone for the tips!
Seems like you could pin/tape the tarp and an appropriate length of string attached at the ends of the arc to a wall and mark it off, as well. Gravity would pull the string into a catenary arc, would it not?
I may be slow... But I sure am gimpy.
"Bless you child, when you set out to thread a needle don't hold the thread still and fetch the needle up to it; hold the needle still and poke the thread at it; that's the way a woman most always does, but a man always does t'other way."
Mrs. Loftus to Huck Finn
We Don't Sew... We Make Gear! video series
Important thread injector guidelines especially for Newbies
Bobbin Tension - A Personal Viewpoint
Looks like I'm going to be the nonconformist, here. Unless you're counting grams, I don't see the significant benefit of cat-cuts. I have a rectangular tarp and get it plenty taught. On a very windy-cold outting last Fall, I was very happy I had the full-coverage of the rectangle. Cat-cuts would have left me way too exposed to the wind. To answer the original posters question: go no cat.
"Every minute outside ... is a good minute!" -> Calvin & Hobbes, 8/1/1993
Did you only buy a piece of sil as big as your tarp? If not practice with the extra.
I'd say go rectangle until you feel comfortable enough with your machine and your fabric to do something challenging. I've made 3 tarps and 4 hammocks and I'm feeling that I'll try to cat cut the next tarp. Just getting the hems and seams straight was a challenge on the first couple. Now I'm feeling comfortable and confident that I can add a little complexity. You can always go back and cat cut a rectangle or sell off that rectangle for the cost of the materials and make a cat cut one. That's what I've done, I'm letting other people pay for my practice while I refine my technique and design. There still getting better and cheaper than if they went and bought one of the cheapo mass produced ones.