Identifying Census Block Groups on a Block Map

Here’s a quick tip that came out of a recent custom data request for figuring out where the Census block groups are when all you have a block map.

Question
“I’m looking at the map right now, and I see that there is a key for Census Tract and Census Block. How does one figure out the boundaries for Census Block Group?”

Answer
“Excellent question – though my guess is that you won’t be happy with the answer.

There are no block group boundaries on the maps. But you can identify the boundaries using the first digit/s of the blocks.

An image is probably helpful. So for Census Tract 502, the boundary of Block Group 1 is the boundary of all of the Blocks that start with 1 (1000, 1001, 1002, etc). See my blue dotted line below.

large

Click to see a larger, clearer version of this map.

Follow Up Question
“One question: In this case, what would the block group be called? I have some data that only break down at the block group level.”

Follow Up Answer
“In most cases (in fact, I can’t think of any case where this wouldn’t be true…but just in case, I’ll say in most cases), the block group name is the first digit of the blocks (Block Group 1 in the example below).”

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