Mercer County, New Jersey Map with Townships and Land Area

How to use this map

The map opens on the Divisions view with all 12 of Mercer County’s municipalities drawn straight from Census boundary files. Click any one to pin it and read its figures: land area in square kilometres, 2020 population and the density that falls out of those two. The tabs above the map change what the shading means. Divisions gives each unit its own colour so you can tell neighbours apart at a glance. Land area shades by size. Satellite drops one metre aerial imagery under the boundaries. Blank outline strips the fill away for printing or tracing.

The figure people misread is land area. Census land area excludes water, so a municipality with a lake or a river frontage looks smaller in the table than it does on the map. Hopewell township is the clearest case here: its boundary encloses 161.21 square kilometres but only 150.4 of that is land, leaving 10.81 as water. If you are measuring something that sits on the ground, use the land figure. If you are measuring something that follows the boundary, use the polygon.

What this county is made of

Mercer County holds 7 townships, 3 boroughs, 1 Princeton, 1 city. Hopewell township is the largest by land area at 150.4 square kilometres and Hopewell borough the smallest at 1.87, which makes the largest roughly 80.4 times the size of the smallest. The median sits at 40.32 square kilometres, so the spread is wide but most units cluster nearer the middle than the extremes.

Trenton is the county seat, which is the New England term for what the rest of the country calls a county seat. Hamilton is the largest by population at 92,297, about 24 per cent of the county total. Trenton is the densest at 4,612.7 people per square kilometre and Hopewell township the emptiest at 116.3, a ratio of about 40 to one. Hopewell borough has 1,918 people.

Across the whole county the density is 666.2 people per square kilometre. That is the number to hold on to, because it explains the shape of the map: a few dense nodes and a long tail of thinly settled ground, with the average describing almost none of the individual units.

Where the data comes from

Boundaries come from the Census Bureau TIGERweb county subdivision layer, which is the authoritative source for municipal geography in New Jersey. Land area is the Census land measurement, which excludes inland water. Population is the 2020 Decennial Census count for each municipality.

The sources are joined on municipality name, and that join is worth checking rather than trusting. The 12 land areas sum to 581.4 square kilometres against a published county land area of 581.4, which agrees to the hundredth. The 12 with published figures sum to 387,340, which is the published county total to the person.

Aerial imagery, and how it is fitted to the map

The Satellite tab pulls one metre aerial imagery from the USGS National Map. One metre means each pixel covers roughly a square metre of ground, so at full zoom you can pick out individual buildings, field boundaries and the line of a farm track. That resolution matters at this scale. The national and state maps on this site use a NASA composite at five hundred metres per pixel, which is fine for showing where New Jersey sits in the country and useless for showing where a municipal line runs.

The boundaries are drawn in an Albers equal area projection fitted to this county, with standard parallels set inside its own latitude range. Equal area matters here because the map is shaded by area: if the projection distorted area, the shading would be lying. The imagery arrives in Web Mercator and is reprojected to match.

A check you can repeat on paper

Take Trenton. Its land area is 19.7 square kilometres and its 2020 population is 90,871. Divide 90,871 by 19.7 and you get 4612.74 people per square kilometre, which is what the panel shows rounded to 4,612.7.

Now check it against the county. Trenton holds 23.5 per cent of the 387,340 people on 3.4 per cent of the 581.4 square kilometres. That gap between population share and land share is the whole story of the map in one comparison.

Mistakes that catch people out

Reading polygon area as land area. The map draws boundaries, and boundaries include water. This county encloses 607.35 square kilometres of polygon against 581.4 of land, so 25.95 is water, about 4.3 per cent.

Assuming a city is bigger than a town. In New Jersey those words describe charter type, not size. Hopewell borough is the smallest unit here at 1.87 square kilometres.

Expecting the average to describe a unit. At 666.2 per square kilometre the county average sits between Hopewell township at 116.3 and Trenton at 4,612.7, and matches neither.

Assuming municipal lines follow roads or rivers. Many were surveyed as charter grants or survey townships long before the roads existed. The straight lines on this map are the original survey, not modern infrastructure.

Related Calculators and Reference

• New Jersey County Map, for all 21 counties and where Mercer sits among them
• United States Map, for the national view and how New Jersey compares
• Massachusetts County Map, for the same treatment applied to another New England state
• US Census Bureau Geography, the source for boundaries, land area and population