I believe it depends on how fast you ping the server: If it's one ping per second (or even slightly faster), they will most likely not care. If it's much faster, they may consider it a DDOS attack by ping flood. It's especially the case if you don't wait for the previous answer before sending the next ping.
It reminds me of the kids who brought Yahoo!, Amazon and some others down to their knees a few years back by flooding them with pings. Since then, yes, ping is considered a potential weapon.
Also, be careful about what part of the network you want to sample. You never know who answers to google.com queries. More accurately, you never where the answer comes from. Chances are it comes from not very far from you (you're being geo-localized) but you can't know for sure. I'd target a smaller organization where you can first identify the location of the server.