Introduction
I heard tell of a Gravity Hill in Forrestfield, in Perth’s eastern suburbs.
I haven’t been out there yet, but it is pretty easy to work out what is going on from topography maps.
Background
These gravity hills, sometimes called magnetic hills, are places where people report that their cars roll uphill. There are dozens of them around the world. Sometimes people even make money off them, but you can only do that if they are in an out of the way place that is hard to find.
SPOILERS: they always turn out to be an optical illusion but the details vary. Sometimes the illusion is caused by the presence of trees that have been tilted by shifting soil or storms. Usually, though, it is due to an inflection in road grade giving the impression of a rise ahead. Usually when you get out of your car, the illusion is destroyed.
Forrestfield Gravity Hill
The Forrestfield Gravity Hill, also called Magnetic Hill, lies between two turns along Holmes Road. It is in the vicinity of 200 Holmes Road.
The stretch of road is about 200 metres long. The effect works when travelling westward.
Here’s an example of a video of someone experiencing this effect.
By comparing this guy’s vision with Google Street Maps I’ve determined his starting location.
I have obtained his altitude (elevation) from the data at Topographic-map.com, which ultimately gets its data from the satellite derived ASTER map.
His starting elevation is approximately 86 metres. He is on a stretch of road with a gradient of about 3%, and he’s rolling downhill.
He starts about 100 metres from the beginning of the turn. The elevation near the start of the turn is 83 metres.
Over the course of the left turn, the elevation decreases about 8 metres in the space of an 80 metre run, ie the gradient is around 10%.
Note that there is foliage behind this turn, obscuring the horizon.
Summary
There are three factors that aid the impression that one is approaching the crest of a hillock in this case.
Firstly, and most importantly, there is a fairly sharp increase in grade at one point in the road, which looks basically the same as a gentle peak in elevation would look to a driver.
Secondly, the fact that is a left turn combined with this sharp grade inflection means that the road past that point is completely hidden from the driver. If the road was straight then the driver would have more visual information about the elevations: as it is, the road seems to “disappear”.
Thirdly, the presence of trees behind the grade inflection means that the driver can’t see the horizon, or anything distant. If the driver could see the horizon, it would be obvious that he is heading downwards.