Does anyone out there know of any metric that anyone has used to actually define what is urban terrain and what is not?

If such a metric exists, I gather it would be either the percent of area settled or developed (which is fairly difficult to measure)or population density per square kilometer (which is relatively easy to measure).

Lowest figures I have on record for city densities are in the range of 7 persons per hectare at Houston, TX. Highest figure I got is 280 persons per hectare in Hong Kong. You can check here for the Newman and Kenworthy graph I adapted to use recently on a book on urbanism I wrote. The graph shows the relationship between density and gasoline use, so I doubt is totally relevant and includes just a handful of cities. Now, if we are talking global warming, this graph is a beauty: it shows that is better to plan the city than use hybrid vehicles, but I'm digressing...

Densities for a few selected major cities:

Notice Houston is "off the mark" as far as urban terrain is involved, but not too far from average for USA: a city like New York has only 30 (persons per Ha) while Los Angeles goes around 34. European cities are in the 75-150 persons per Ha mark, while cities in Asia go from 160-170 for Tokyo and Singapore to the 280 maximum mentioned for Hong Kong. All data is for 1989: I believe, intuitively, that densities must have increased in the last 20 years in those cities, unless you take in account the trend toward the periphery shown in the last graph of this (sorry!) long post.

To show how low is the Houston figure, I can tell that in Colombia, the country where I live, the most densely populated rural area has 3.5 persons per hectare (Itagüí, on the outskirts of Medellín) and only 4 of 1088 counties in all the nation have over 3 persons per Ha in the rural area, so if you go lower than Houston you have, essentially, a collection of small farms, although Houston has a business district center that rural Itagüí lacks.

Extracting for 64 cities in Colombia for which I have maps showing the approximate urban perimeter (scale 1:500.000, which gives me a larger error for small towns) the information from the 1993 Census, I got the following graph (in red, national average of 150 persons per Ha):

Notice also that most of the mega-cities created in the world in the last 25 years are in the developing world. The balance of cities has "moved south", as this other graph, taken from BBC News shows (same source: "Urbanism and Construction", Pabón & Pabón, Bogotá, 2007, which means I constructed it, based on BBC data):

Cities over 5.000.000 in 1955:

Cities over 5.000.000 in 2015:

I guess size is also important to separate a city from a town: for example, under 50.000 inhabitants probably doesn't allow for an area big enough to be defended in "urban mode", no matter the density, but this is just an opinion. If cities were perfect circles and had an average 100 persons per Ha, this 50.000 people figure would mean a city radius of 0.8 miles (1.3 Kilometers), which gives you a meager 13 to 15 blocks of "defense depth": perhaps some urban combat specialist can give an opinion about how long you can sustain a position given this "thickness" of your line of defense, before you fall under cross fire, which I believe is a typical problem of a surrounded army group. Roughly speaking, your defense depth would be proportional to the square root of your population.

Finally, the smaller the city, the larger the density in the central blocks and the more sparsed is the population of the perimeter: here you have an example of population distribution that shows (same source, Pabón & Pabón, taken from Jane Jacobs's, “The Economy of Cities”, Vintage, 1970, ISBN: 039470584X) how smaller cities have less people living in the periphery. The light gray bars are numbers for an average city, the darker ones for the mega-cities of the world. Vertical scale shows the percentage of population. The bars, from left to right, show the percentage of population living at less than one mile, one mile to three miles, three to five miles and over five miles:

City population distribution:

http://cgsc.leavenworth.army.mil/carl/download/csipubs/Block/intro_Block%20by%20Block.pdf

and
http://cgsc.leavenworth.army.mil/carl/download/csipubs/spiller3/intro.pdf

may shed some light, or just add to the confusion!.

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Originally posted by Nick-:
Intresting question and i guess youll find a number of different answers.
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Both texts give an indication of what urban combat is, but not very clearly what's an urban evironment. Anyway, given that, according to these references:

"Not only are cities themselves living systems but also they are composed of supporting interstitial systems: water, food, power, communications, transportation, manufacturing, economic, commercial, entertainment, and many others.

...

Understanding the framework of functional systems then provides a parametric baseline for tactical operations against villages and towns and for operational- or strategic-level attacks against large cities. This
is in contrast to the little, mock villages created at many U.S. military installations as training sites for urban operations."

And given that at Stalingrad, "German military doctrine was based on the principle of combined-arms teams and close cooperation by tanks, infantry, engineers, artillery, and ground-attack aircraft. To counter this, Soviet commanders adopted the simple expedient of always keeping the front lines as close together as physically possible. Chuikov called this tactic "hugging" the Germans", which negates supporting fire. Besides, "The Germans, calling this unseen urban warfare Rattenkrieg ("rat war"), bitterly joked about capturing the kitchen but still fighting for the living-room... German tanks became useless amid heaps of rubble up to 8 meters high. When they were able to move forward, they came under Soviet antitank fire from wrecked buildings."

The text you provide insist on the same concept: "All is well in one city block, but all hell rages two blocks over. The first floor is cleared here, but why are the miserable illegitimates dropping cocktails on us from two floors above and firing rocket-propelled grenades at us from the cellars below?"

Another quote from these two articles that seems relevant is "Only one assumption could be made with any sort of confidence: once ground forces were introduced, a significant part of their duties would be performed not in the open countryside but in areas that could to some degree be characterized as urban. Some such areas might be very small, no more than a village perhaps, with a population numbering in the tens. Some might be towns with only a few thousand inhabitants. Others might be much larger municipalities, with populations running to the tens of thousands. The question naturally arose: to what degree was the US Army prepared for this mission, ill-defined as it was at that particular time?"

Also I find interesting that "... Behind this guidance lies the suspicion that weaker adversaries in the future would choose as their preferred battleground the vast urban
agglomerations of the world..." and that's why I thought relevant to show how huge cities have been created in the last 50 years in the "south".

Essentially, what I see is a landscape where mobility and firepower are negated to the attacker, taking away from conventional armies most heavy artillery and air support. Besides, a city huge food reserves, communication and water systems provide a shelter for defenders that does not exist in open country. No wonder that "... at the Grain Elevator, a huge grain-processing complex dominated by a single enormous silo, combat was so close that Soviet and German soldiers could hear each other breathe. Combat raged there for weeks. When German soldiers finally had reduced the opposition they found approximately 40 bodies of Russian soldiers."

I find notable that not only you lose artillery support (the bass of the orchestra) but you are in grave danger of losing politic support by population, reading about how Fallujah changed from a city where "... the new mayor of the city, Taha Bidaywi Hamed, selected by local tribal leaders—was strongly pro-American... A Fallujah Protection Force composed of local Iraqis was set up by the US-led occupants to help fight the rising resistance" to a site where "Operation Phantom Fury resulted in the reputed death of over 1,200 insurgent fighters. Approximately 100 American Marines were killed, and over 1,000 wounded."

So, to answer Chris question, I would say that this kind of defense is not feasible when urban densities fall below 20-30 persons per Ha, which seems reasonable enough to characterize this kind of combat: this correspond to a sparsely populated "normal" city, with low density. You can expect a typical urban terrain to fall in the 100 persons per Ha range in most of the world. I'd say that definitely you're in rural terrain when you have 3-5 persons per Ha, that is, only one house per block.

With population densitys being higher than before in human history, it stands to reason that there is more urban terrian around than before if you use a metric that has people per sq klm etc. But a sq klick of multi story stone/concrete/reiefourced with structual steel for load bearing buildings prents a different mil set of problems than does the same sq klick with single story, even more so if the type of materials used in significntly differnt, one of the earlist citys, in the ME had all the houses linked with doors facing internaly into a courtyard, and the roof tops used to observe/fight from and only entry points were at the cardinal points so there was only 4 points of entry into the compound/city.

Scottish round towers had the stair case run in manner so that to climb up presented the clmber with his right hand close to the wall, in an age of combat being swords, thios of course presents a problem in manouvering your arm, and the advantage lies with person comming down as he has afullwer range of movement to strike with.

Ask an archeologist and he might go along with that, but then again he might go with hearths density to define urban.

Ask a mil person in the first century BC, whats urban, and he will probaly tell you its what inside the city walls, ask someone today, and the city walls have no meaning at all and defining the limits of the urban area.

You could liken it to a question of context, what was urban in the 30 years war has little mil context to what was Urban in the Middle east pre christain era, still less to say Grozny in recent years conflicts, because of the differnce in mil technology and implemtation of use, so any metric that defines Urban without taking into acount the purpose of the excercise, in its proper context is off to a bad start.

US Indian plains wars, if you use pop density as the determination of urban terrain, you get the indian villages as urban terrian, with the cavalry trying to get into it at every opurtunity.

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Originally posted by Ciro Pabón:
If cities were perfect circles and had an average 100 persons per Ha, this 50.000 people figure would mean a city radius of 0.8 miles (1.3 Kilometers), which gives you a meager 13 to 15 blocks of "defense depth": perhaps some urban combat specialist can give an opinion about how long you can sustain a position given this "thickness" of your line of defense, before you fall under cross fire, which I believe is a typical problem of a surrounded army group. Roughly speaking, your defense depth would be proportional to the square root of your population.

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Ciro. Just reread your entire post. There is a lot to chew on here. This is a very interesting statement, especially when combined with the one below.

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Finally, the smaller the city, the larger the density in the central blocks and the more sparsed is the population of the perimeter: here you have an example of population distribution that shows (same source, Pabón & Pabón, taken from Jane Jacobs's, “The Economy of Cities”, Vintage, 1970, ISBN: 039470584X) how smaller cities have less people living in the periphery. The light gray bars are numbers for an average city, the darker ones for the mega-cities of the world. Vertical scale shows the percentage of population. The bars, from left to right, show the percentage of population living at less than one mile, one mile to three miles, three to five miles and over five miles:

City population distribution:

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Now, I gather this means that one could produce some form of model of cities, addressing expected high density, medium density, and outlying areas, based upon population, economics and settlement habits. From there....I have no idea how to mix and match it with anything else.