Look at a map — Indianapolis' downtown streets are tilted. Why don't they point due north?

Open Google Maps. Zoom out until you see most of the inside of the I-465 loop around Indianapolis.

Now look closely at just the portion inside the inner loop of I-65 and I-70.

Does the whole grid of streets seem to be tilting?

It's ever so slight, but it's real, and not just a glitch in mapping software.

Documentation to explain the aberration is, apparently, rare. Multiple sources reached by IndyStar pointed to one piece of text they know of: J.P. Dunn’s 1910 book, "Greater Indianapolis," Vol. 1.

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In what's now known as "Mile Square," bordered by East, West, North and South streets, Dunn writes, "the streets do not run direct to the points of the compass, as commonly supposed; they bear about two and one-half degrees east of north, and south of east, owing to variation in the magnetic needle."

He goes on: "Most of the streets in the additions, outside of 'the donation,' follow the section lines, which were run on the basis of the true meridian."

Let's break that down.

The 'Donation'

When Indiana became a state in 1816, the federal government granted the state 4 square miles of public land, known as the "Donation," to build its capital. An appointed commission selected the site where the White River meets Fall Creek.

In 1821, commissioners hired Alexander Ralston, who had just worked on the planning for Washington, D.C., and Elias Pym Fordham to lay out the city of Indianapolis.

Ralston doubted the city would ever be as large as 4 square miles, according to the "Encyclopedia of Indianapolis," so he drew a map of just 1 square mile of neat blocks quadrisected by diagonal streets radiating from a center circle. The Mile Square.

At the top of Ralston's plat, or map, is a compass rose seemingly indicating that Meridian Street points north.

"And that has always been my assumption, and probably the assumption of most folks," IUPUI history Professor Emeritus Robert Barrows said via email. "But, no."

According to Dunn, they are instead oriented toward "magnetic north," and streets outside the 4-square-mile donation were built according to "true north."

Magnetic vs. true north

Real compass needles point toward "magnetic north," because the location of this north is based on the earth's magnetic field, which is influenced by its iron core, Owen Dwyer, chair of IUPUI's geography department, said via email.

"That core is largely molten and sloshes around in an uneven, lumpy kind of way," he wrote. "As the core sloshes around, the magnetic field responds by following it. As a result, the magnetic north pole moves."

The Earth's "true" or geographic north pole is a fixed spot, where the Earth's axis meets the northern hemisphere.

So, a compass today points in a slightly different direction than it did 200 years ago. The magnetic north pole has been moving away from Canada and toward Russia since then, Dwyer said.

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Outside the original donation, the streets seem to tilt in a straighter fashion, following "true" north, which never changes.

According to the National Oceanic and Atmospheric Administration, the difference between magnetic north and true north in Indianapolis changes by just over 3 miles every year.

"Given that magnetic north moves around, geographic north is the stable, easier-to-work-with version of the cardinal directions," Dwyer wrote.

Not the original BLM plan

It was apparently the intention for the entire city to follow true north, but it's unclear why the Mile Square planners deviated.

The "section lines," Dunn notes, were drawn according to true north.

Those section lines were there before the Mile Square was mapped out, Bryan Catlin, technical supervisor in the Marion County Surveyor's Office wrote via email.

In 1819, a surveyor from the General Land Office — a federal agency that was responsible for public lands and became the Bureau of Land Management in the 1940s — laid out the 6-square-mile township that the Mile Square falls within. The following year, another surveyor divided the township into 36, 1-square-mile sections. Part of this township was the "Donation" the federal government gave to Indiana.

The GLO surveyors would have used magnetic compasses, but corrected their "section lines" to true north according to variations they calculated from looking at the stars, Catlin wrote.

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While Ralston and Fordham were platting Mile Square in 1821, markings from those 1819 and 1820 GLO surveys, such as wooden posts and blazed lines, would certainly have been "very visible" through the woods, he said.

Why they chose to deviate from those "section lines," no one reached by IndyStar knows for certain.

Maybe they wanted to configure the Mile Square in a location that avoids the White River's floodplain, but that's only one of Catlin's guesses.

What we know for certain: We can't un-see it.

Contact IndyStar transportation reporter Kayla Dwyer at kdwyer@indystar.com or follow her on Twitter @kayla_dwyer17.

This article originally appeared on Indianapolis Star: Downtown Indianapolis: Google maps show streets alignment slightly off