Just came across this map for chaining.
Original source:
Yep, I remember reading about this back in the original Sinnoh games. This is, in fact, why every land route in Sinnoh has at least one 7x7 patch of tall grass. It's not to annoy you; it's to give you an easy Poké Radar location.
A bit of further advice if you want to chain: using the Poké Radar in the midst of a chain will not break it. Instead, it resets the possible locations. Hence, the best way to have a longer chain is NOT to be in the middle of that 7x7 patch of grass, but off to one corner without being at an edge or corner tile. This is to ensure you always have at least one rustling tile (the Poké Radar CAN choose tiles not part of the tall grass, and if all four are outside it, the chain automatically ends) and the highest chance that the one at Ring #4 is also surrounded by tall grass, allowing the chain to continue. If no tiles shake at Ring #4, reset the Poké Radar and keep doing so until it happens. I used this technique to get two shiny Nosepass in one go back in the original
Diamond game--the longer the chain, the higher the odds of a Shiny Pokémon, topping out at a chain length of 40 when your odds were 1 in 200 (meaning roughly 1 in 50 for each Poké Radar reset).
If you use the Poké Radar at the location that player character is standing, the shaking grass at Ring #4 will be a corner or edge tile, resulting in a high chance the next one will all be outside of that patch of grass and thus ending the chain.