“Canvasses” occur at violent crime scene. During a canvass, cops go around the neighborhood looking for witnesses and taking interviews.
In general, most canvasses will be about 2 blocks in all directions around the crime scene. BUT, they can go further. Some go for a half mile, again in all directions.
…but its not over after the first pass. If a good lead detective is working the case, there will be another canvass the next day, at the same time when the crime occured. Then a week later at the same time, and a month after that.
The “common” perception of beat cops is that canvasses are a pain in the ass. They don’t get any credit for them. Many times leads will send out just anyone out to go canvass. The average cop just wants to get it over with.
Many modern cops are “badge-heavy.” No one wants to talk to cops like this. So naturally, they don’t find any witnesses or they find only reluctant wits.