UPDATE 2024/01/04: This post originally appeared on my Civility and Truth Substack newsletter. I’ve moved it to my main site in an effort to collect all of my writing in one place.

I’m back again with more population density maps, this time for Ellicott City—or, more precisely, the Ellicott City Census Designated Place or CDP.

The map above shows population density variations as of the 2010 census for the various census block groups that are wholly or mostly within the Ellicott City CDP. There are 34 such census block groups, compared to 54 census block groups for Columbia or 154 census block groups within the county, with Ellicott City thus accounting for about a fifth of Howard County. (The 2010 population of 64,245 for these census block groups is also almost a fifth of the county’s total 2010 population.)

The main take-away from the map above is that most of Ellicott City has pretty much the same population density: it has less multi-unit housing than Columbia, and fewer large lots with single-family homes than western Howard County. (The major exception is the area east of US 29 between US 40 and I-70, which contains a number of apartment complexes.)

This is confirmed by the histogram above, which shows the distribution of density among the various Ellicott City census block groups in 2010. Most of the CDP was between 1,000 to 4,000 people per square mile, with only two census block groups out of 34 having higher density. The overall population density for Ellicott City in 2010 was 2,234 people per square mile, about a third less than the overall population density for Columbia.

The next map shows changes in population density in Ellicott City between the 2010 census and the 2017 American Community Survey 5-year estimates (which reflect surveys done in 2013 through 2017). Even over this relatively short period we can see a significant decrease in density in the area between US 40 and I-70 west of Rogers Avenue (which includes a good chunk of Patapsco State Park).

There was also almost a doubling of population density in the area north of old Ellicott City and south of US 40. I don’t know if the latter is due to new development or to changes in household size. However it’s worth noting that based on the 2010 population density map compared to the map for 2013-2017 immediately above, that area was originally less densely populated than most of Ellicott City, and whatever changes occurred brought it up to a “typical” density for the area.

As I did for Columbia, I also mapped density variations at the level of census blocks. This uses population data available for the 2010 census but not for the American Community Survey.

This map is most notable for showing areas of Ellicott City that have very low population densities. These include retail areas like the Long Gate shopping center north of MD 100 and east of US 29, park areas including Patapsco Valley State Park, Meadowbrook Park, and Centennial Park, and golf courses like the one at Turf Valley.

Note that as with Columbia some areas appear on this map that were not on the prior maps. These reflect census blocks that are in the Ellicott City CDP, but that are in census block groups that are mostly not in Ellicott City. The most notable example of this is the Turf Valley resort.

There is also a set of Ellicott City census blocks west of Centennial Lane that appear to be almost disconnected from the rest of the CDP. These appear to be associated with the soccer fields and church at Covenant Park, with Centennial High School and Burleigh Manor Middle School another “low-density” area just north of there. (Recall that “low-density” in this context refers to the size of the residential population in a given area, not how built-up the area is.)

You can find the code and data behind this post, as well as more Ellicott City population statistics, in the document “Ellicott City, Maryland, population density.”

That’s all for this week. A reminder: if you find these posts interesting and useful please tell other people about them and encourage them to subscribe to the Civility and Truth mailing list. Having readers who care enough to subscribe helps motivate me to send these posts out on a regular basis, and the more readers I have the more motivated I’ll be. In the meantime, thanks for reading this post!

UPDATE 2024/01/04: This post originally appeared on my Civility and Truth Substack newsletter. I’ve moved it to my main site in an effort to collect all of my writing in one place.

Hey! I finally managed to figure out to get a list of census block groups or census blocks for the Columbia CDP. (As a reminder, “CDP” or “Census Designated Place” is US Census Bureau jargon for a population center that’s unincorporated.) So now I can bring you some density maps that zoom in to focus on just Columbia as opposed to all of Howard County.

The map above shows population density variations as of the 2010 census for the various census block groups that are wholly or mostly within the Columbia CDP. (Some census block groups contain only a small portion of the Columbia CDP. I omitted them from the map.) There are 54 such census block groups, as compared to 154 census block groups within the county. Thus from this point of view Columbia accounts for about a third of Howard County.

As I mentioned previously, census block groups are a nice “not too large, not too small” subdivision. In 2010 the least populated census block group in Columbia contained 645 people, while the most populated census block group contained 3,632 people. The smallest Columbia census block group covered an area of 0.12 square miles (about 79 acres), while the largest block group covered an area of 3.11 square miles.

A typical census block group is considerably smaller than a Columbia village: since there are nine Columbia villages, each village would contain about 6 census block groups on average if they were equally distributed.

In the map above I’ve shown more roads than in my previous maps, to help orient you vis-a-vis the various parts of Columbia. I thought about also superimposing the boundaries for the various Columbia villages (data that’s available on the Howard County GIS site), but ran out of time to make this work.

The main take-away from the map above is the areas of Columbia that are relatively high-density vs. relatively low-density. Relatively high-density areas include portions of Harpers Choice and Owen Brown villages, presumably due to apartment complexes there. In contrast the Columbia Gateway area is primarily office space and thus low-density in terms of residential population.

The histogram shows the distribution of density among the various Columbia census block groups in 2010. Density varied from a low of 761 people per square mile to a high of 13,285 people per square mile, a difference of over an order of magnitude. Overall population density for Columbia in 2010 was 3,187 people per square mile.

As a comparison, the lowest density Howard County census block group in 2010 had 151 people per square mile, and the highest density block group had 15,181 people per square mile. Overall population density for the county in 2010 was 1,144 people per square mile. Thus the least-dense Columbia census block group was over five times as dense as the lowest-density Howard County block group, and Columbia as a whole was about three times more densely populated than the county as a whole.

The next map shows changes in population density in Columbia between the 2010 census and the 2017 American Community Survey 5-year estimates (which reflect surveys done in 2013 through 2017). Even over this relatively short period we can see significant decreases in density in the area of Harpers Choice village off Harpers Farm Road in northwest Columbia, and a increase in density in the area just south of there, north of Little Patuxent Parkway and east of Cedar Lane. I don’t know if there’s been any change in the total number of housing units in those areas, so my initial guess is that these changes are due to changes in household size: older children moving out on the one hand, and more young children on the other.

For my final map I decided to map density variations at the level of census blocks. This uses population data available for the 2010 census but not for the American Community Survey.

Census blocks are very small: there are 1605 census blocks in the Columbia CDP, with an average area of 0.02 square miles (about 13 acres). In 2010 over half of the census blocks contained no people at all, and the average population of a block was only 62 people; the largest block contained 2,696 people.

Because of the small size of census blocks the corresponding population density can be very high if the block mainly contains apartment complexes. In 2010 there were several census blocks in Columbia with population densities over 50,000 people per square mile, and a few over 100,000 people per square mile. At the other end of the spectrum the majority of census blocks in Columbia contain no people and thus have a population density of zero.

Because of this wide distribution of population densities I don’t think the block-level map is that useful for looking at population densities in residential areas. However on this map it’s very easy to pick out the parts of Columbia that are devoted to office, retail, or industrial uses (in dark blue). This includes the Columbia Gateway and Dobbin Road areas on either side of Snowden River Parkway, the areas along Broken Land Parkway, the Mall at Columbia and the future Merriweather District, and the area around the hospital and Howard Community College.

UPDATE: I was able to resolve the problems that were preventing me from publishing the underlying code and data for this post. See the document “Columbia, Maryland, population density.”

That’s all for this week. A reminder: if you find these posts interesting and useful please tell other people about them and encourage them to subscribe to the Civility and Truth mailing list. Having readers who care enough to subscribe helps motivate me to send these posts out on a regular basis, and the more readers I have the more motivated I’ll be. In the meantime, thanks for reading this post!

UPDATE 2024/01/04: This post originally appeared on my Civility and Truth Substack newsletter. I’ve moved it to my main site in an effort to collect all of my writing in one place.

This is a brief follow-up to last week’s post. I had hoped to be able to take a closer look at density variations in Columbia and Ellicott City. However I haven’t yet found a good way (at least via the Census API) to get a list of census block groups or census blocks for the Columbia and Ellicott City CDPs. (“CDP” or “Census Designated Place” is US Census Bureau jargon for a population center that’s unincorporated.)

I also happened to think about whether the census block group boundaries had changed from 2010 to 2017. After looking at this I concluded that they probably had not, but this took a while to nail down.

The one new thing I did was to produce a map of density variations based on the quintiles the various census block groups fell into. In other words, in the map above the census block groups in dark blue (quintile 1) are the 20% of all census block groups with the lowest population density in 2010, while the census block groups in yellow (quintile 5) are the 20% of all census block groups with the highest population density in 2010. The other areas in light blue, green, and orange (quintiles 2, 3, and 4 respectively) are intermediate between those two groups.

I think this map does a slightly better job of letting you tell at a glance which are the most dense and least dense parts of the county, as well as which areas are roughly in the middle in terms of density.

Here’s a similar map using the 2017 ACS 5-year estimates, which cover the 2013-2017 timeframe. At first glance I can’t see any differences between this map and the prior map. This means that even though some areas of the county may have experienced changes in population density between 2010 and 2013-2017, the changes weren’t large enough to make any real difference in the overall density picture.

I’m going to try again to do density maps for Columbia and Ellicott City. In the meantime, see the revised version of “Howard County density trends by census block groups” for the code behind the maps above.

That’s all for this week. A reminder: if you find these posts interesting and useful please tell other people about them and encourage them to subscribe to the Civility and Truth mailing list. Having readers who care enough to subscribe helps motivate me to send these posts out on a regular basis, and the more readers I have the more motivated I’ll be. In the meantime, thanks for reading this post!

tl;dr: Some areas of Howard County are over a hundred times more densely populated than others.

A long-time theme in writings about Howard County is the distinction between the more densely populated suburban and semi-urban areas like Columbia and the less densely populated rural areas in the western part of the county. This has implications for issues from political affiliations to school redistricting, and of course for affordable housing as well.

In this post I’m going to ignore those issues though, and just look at the simple facts about density variation across the county. The map above shows density variations as of the 2010 census—a data source I chose because it contains accurate population counts at a fairly fine-grained level.¹ The map shows population densities (people per square mile) for each of the census block groups within the county.

A census block group is a geography defined by the US Census Bureau that is one level below a census tract. There are currently 154 census block groups in Howard County, compared to 55 census tracts. The smallest geography in US Census data is the census block, one level below the census block group. There are currently 4,845 census blocks defined for Howard County.

Census tracts are relatively large, about 6,000 people or so on average in Howard County. On the other hand, census blocks are too small: more than half of all census blocks in Howard County contained no people at all in the 2010 census.

Census block groups are a nice “not too large, not too small” subdivision of the county’s overall area. In 2010 the least populated census block group contained 645 people, while the most populated census block group contained 3,632 people. The smallest block group covered an area of 0.1 square miles (about 64 acres), while the largest block group covered an area of 13.4 square miles.

A typical census block group is thus comparable in both size and population to a traditional village or small town. It is large enough to be a recognizable “place,” but small enough to have its own identity distinct from that of other places in the county.

What about density? One of the most surprising things to me in doing this analysis was the wide variation in population density across the county. Population density in 2010 varied from a low of 151 people per square mile to a high of 15,181 people per square mile, a difference of two orders of magnitude. In comparison, overall population density for the county in 2010 was 1,144 people per square mile (287,085 people divided by the county land area of 251 square miles).

The map above is a variation on the first map. It is based on the same 2010 census data, but divides the census block groups into five groups (or “quintiles”) of 31 block groups each (or 30, for the highest quintile). This map shows much more clearly that almost all of the census block groups with the highest population density are in Columbia and eastern Howard County, and almost all of the census block groups with the lowest population density are in western Howard County.

(Some of the major exceptions are areas like Columbia Gateway and the light industrial districts east of I-95 that have little or no residential construction. Here “low population density” is not the same as “not built up.”)

As shown in the above histogram, and also in the first map above, there are many census block groups in Howard County that had a population density of less than 500 people per square mile in 2010. At the other end of the spectrum a few census block groups had population densities of well over 10,000 people per square mile.

The largest number of census block groups fell into the range of 2,000–2,500 people per square mile; the typical (median) block group had a population density of about 2,400 people per square mile. (This is different from the overall population density of Howard County of 1,441 people per square mile quoted above.)

To help think about what these numbers mean, consider a square mile, about 640 acres. Suppose we have a few neighborhoods of single-family houses on 3- to 4-acre lots, with three or four people per house. With 50–100 total houses we have a total of 150–400 acres and 150–400 people. Throw in two or three 100–150 acre farms plus road surfaces and open spaces and you’d have a typical Howard County rural census block group with a population density of 200–500 people per square mile.

Now suppose instead we have 0.1 square miles, about 64 acres, occupied by five or six apartment buildings with 50–100 units each, with two to three people per unit. Now we have 500–1,800 people total in that 0.1 square mile area, for a total of 5,000 to 18,000 people per square mile—in other words, a typical semi-urban setting in Columbia or eastern Howard County.

How is population density in the various parts of Howard County changing over time? It’s hard to get a good picture of this in between censuses, because the available population estimates are from surveys taken over multiple years (from 2013 through 2017 for the latest available data) and have fairly high margins of error at the level of census block groups (up to 30% or more above or below the estimates themselves).

The map above is an attempt to show density changes from the 2010 census forward, using the American Community Survey 2017 5-year estimates. As with population density itself, there is wide variation in population density changes.

A few areas stand out as having significant increases in population density, from 50 to 100%. These appear to be include the Maple Lawn Farms development in Fulton as well as adjacent neighborhoods south of MD 216, areas along US 1 south and north of MD 175, and areas near downtown Ellicott City.

Other areas apparently experienced decreases in population density. Assuming that the number of housing units did not decrease in those areas, this likely was caused by the number of people per household decreasing, for example due to children leaving families and “empty nesters” remaining. (Additional census data, for example on household size and the ages of household members, should be able to confirm or refute this idea.)

To sum up: we may argue about how the density divide in Howard County came about and what it all means, but I don’t think there’s any dispute that it exists. It is especially clear in western Columbia, where the drop off in density west and north of MD 108 and (to a lesser extent) south of MD 32 is particularly dramatic. It’s also apparent that new development and demographic changes in family size are having disparate impacts across the county.

Combining this density data with information on socioeconomic and political variables could uncover some interesting patterns. Hopefully I’ll have time in the future to look at this.

Further exploration

For more on how I created the maps and histogram above, see the following:

• “Howard County density trends by census block groups” shows the R code used to produce these and other graphs.
• My hocodata code repository includes copies of the R Markdown files for this and another analyses. (Look in the “affordability” subdirectory.)
• If you sign up for a free account on the Rstudio.cloud service you can open and make a copy of my hocodata project for this and other analyses, and try your hand at it yourself. (Again, look in the “affordability” subdirectory, and check out the RStudio primers to learn how to use the system.)

I also did two other articles focusing specifically on population density in Columbia and Ellicott City for my (now deprecated) Civility and Truth newsletter.