Tag Archives: Topographic Maps

Topographic Map(s) of Heard Island, and a Big Landslide

Heard Island Map, 1985.  Image credit: excerpt from the Division of National Mapping.
Heard Island Map, 1985. Image credit: excerpt from the Division of National Mapping.

A few days ago, I posted about topographic maps, including a discussion of how a small army of small surveyors made one of my local park. At Heard Island, surveying isn’t a walk in the park.

Many maps have been made of Heard Island, showing the topography and general geographic features of the island, and sometimes including the locations of major macrofauna (penguins, elephant seals, etc.).[1] An excerpt I made from one produced in 1985 is shown above. Although there are more recent maps available, including maps with higher topographic resolution, this one is more visually illustrative of the landforms.

Maps of Heard Island are difficult to produce, in part because there is a dearth of high-resolution, high-quality data. In most parts of the developed world, detailed topographic maps are made not through boots-on-the-ground surveying but by airborne LiDAR. For instance, aerial imagery and LiDAR provided very useful data for understanding the Oso landslide in Washington state. However, aerial flights over Heard Island are much less frequent, and mapping efforts there come without the obvious benefits to the local populace.

LiDAR map near the Oso landslide (red region at right), and a larger landslide complex (red region at center).  Image credit: Dan McShane.
LiDAR map near the Oso landslide (red region at right), and a larger landslide complex (red region at center). Image credit: Dan McShane.

Surveying the whole island by foot at high detail is untenable, because the area is quite large, the terrain difficult, and the weather inclement, even in the summer. However, portions have been mapped by hand (and theodolite).

But perhaps the biggest challenge Heard Island presents to cartographers is the rapidity of its changes. Volcanic eruptions can add new land to the island, or make parts higher. Glaciers can carve out the rocks and leave them as till, sometimes in the ocean, sometimes in the lagoons, and sometimes as moraines on the land. Not only can the glaciers carve out the rocks, but as less snow accumulates on the glaciers than is lost to melting, the glaciers will retreat. This opens up new land which before had been covered in ice. Stephenson Glacier, on the southeast corner of Heard Island, has retreated significantly in the last 60-70 years, revealing a great deal of new terrain.

Steep slopes and the very wet environment (lots of snow and rain) lead to very high rates of erosion. Outwash channels from the glaciers can carve into the rock and transport sediment into lagoons and near-shore areas.

Finally, there’s another agent of change: landslides. Take a look at the LiDAR image above, showing the landslide region. Now take a look at the southwest portion of the Heard Island shown at the top of the post. The curving crest along the north and east sides of the volcano, as well as the ridge extending to the south-southwest are interpreted to be the boundary (technical term: scarp) of a debris avalanche (a landslide-like process).[2]

Taken as a whole, these processes change the landscape significantly on a decade-to-century timescale, if not even more rapidly. This is why making maps and keeping them current is so valuable: it give us a way to see how the landscape is changing over time. Perhaps the upcoming Heard Island Expedition will do some mapping and be able to provide updates which reflect the latest changes at Heard Island.

[1] https://www1.data.antarctica.gov.au/aadc/mapcat/list_view.cfm?list_id=1, accessed Feb. 6, 2015. Free registration required for map download.

[2] Quilty, P. G. & Wheller, G. 2000; Heard Island and The McDonald Islands: a Window into the Kerguelen Plateau. Papers and Proceedings of the Royal Society of Tasmania. 133 (2), 1–12.

Geoscientist’s Toolkit: Topographic Maps

Topographic map, excerpt from the USGS 7.5' Series (Winona West Quadrangle)
Topographic map, excerpt from the USGS 7.5′ Series (Winona West Quadrangle (31 MB PDF))

Topographic maps (sometimes just “topo maps”) can tell a lot about a place. They record the varying heights of land, from which inferences can be made about those places and their geology. For instance, despite the claims many people make about Minnesota being a flat place, the map above shows something quite different. Sure, there are no 4 km tall peaks here, but this map contains a 500′ embankment, and many stream channels cutting 300′ down from the hilltops.

With the right perspective, understanding a topographic map is fairly straightforward. Let me tell you a story about topo maps.

When I was in eighth grade, my science teacher, Ms. Fuller, was teaching us about maps and mapping. Rather than just looking at maps, we were going to make maps. On a cool, cloudy fall day, we all loaded onto a school bus for a field trip to a nearby park with a lake in it. The students had been divided into teams of three, and each was given a pair of metersticks and a long tape measure.

As we walked along the path at the water’s edge, every so often (50-100′) a team would be assigned to measure a profile up the hill. One person, Alice, would hold a meterstick upright at the path, and another, Bob, would walk directly away from the lake until his feet were level with the top of Alice’s meterstick. Here, Bob would hold his meterstick upright. The third team member, Eve, then measured and recorded the distance between the two metersticks, from the top of Alice’s to the base of Bob’s. Once that had been done, Alice would make her way through the brush, past Bob, and up the hill until her feet were level with the top of his meterstick. The distance between them was measured, and the process repeated until they had gone ~75 m away from the lake and reached the top of the hill and the edge of the park. If they had extra time, they measured another profile from a different location around the lake.

Back in the classroom, Ms. Fuller compiled the profile data into a topographic map, and added in bathymetric (depth beneath the lake surface) data from an outside source (pardon the pun). You may think that just took the interesting part out of the exercise, but let me assure you, it did not!

The following day in class, we were given a large (18″x18″?) printed copy of the topographic map. On it was a North arrow, and a bold line indicating the edge of the lake. Now the challenge was to build a model of the lake and its surroundings.

We spent the day cutting out pieces of cardboard to match the contour lines. Or rather, we cut the insides out of 18″x18″ cardboard pieces, since the lake area was generally bowl-shaped. Each layer represented a certain amount of height, probably around 3 m. In another class period or two, we had our cardboard pieces fully cut out and glued together, giving us a 3D model of the area. All that was left was to make it look nice. The water was painted one color (typically blue or white, depending on whether we were feeling wintry), and the land a different color (usually green). North arrows were added (for full points), as were horizontal and vertical scale bars (the two scales being different).

Contours on topographic maps represent places where the surface of the Earth passes through a geometric plane for a certain elevation. It’s a slice. Give the slice a width, such as the thickness of corrugated cardboard, and by putting contours together you can arrive at a model of the area.

Places where contour lines are close together are very steep: there is a great change in elevation in a short distance. Areas with few contour lines and large spacing between them are quite flat.

If you have a moment, find your local topographic map, and see what it looks like (US topo maps can be downloaded from the USGS). Alternately, you can enable the terrain overlay on Google Maps (in map mode, not satellite), but you don’t get the excitement of the quadrangle names, the 1:24,000 scale, and the magnetic north arrow.