By Dan May
On Our Land
Dan May
Navigating from point A to point B has never been easier since tools like Google Maps, Waze and others have appeared. Most of us rely on them when planning a trip, driving and rerouting around traffic slowdowns or using mass transit. I even use them when walking in unfamiliar cities.
I prefer Google Maps, in part because I like the features that are embedded in its desktop version – especially a growing number of varied map views and interactive tools. While driving and using the cellphone app, I select the default view with a real-time traffic overlay. But at home on a large monitor, I dwell on terrain and satellite views.
Terrain maps are shaded relief maps that highlight features with noticeable slope changes, like ridgelines. They are computer generated using elevation data with no vegetation or structures present and artificially shaded by assuming the sun is illuminating the ground at a low angle. The source for both default and terrain maps is the National Map, a digital repository prepared by the US Geological Survey that describes the landscape and cultural features of the US.
Satellite views combine multiple types of imagery of the land surface from specific dates and times as captured from satellites, as well as flight-based aerial photographs. Google satellite views also show bathymetry (depth measurement) beneath the oceans prepared from sonar data. I appreciate the underwater data, even though I know an Uber driver can never take me to the edge of the continental shelf.
Google does not collect imagery data but displays compilations from many government and private sources, both from the US and other countries. There are more than 10,000 satellites currently orbiting Earth, and while most are for communication, several thousand continually observe the land, sea and atmosphere and stream that imagery to ground-based data centers.
Some data is proprietary or classified for military use, but much is available to the public. In Google’s case, a firm called TerraMetrics compiles its wide-area satellite images.
There also are state repositories of aerial photographs of land surfaces dating back a century that provide more detailed closeup information. These photos, along with satellite images, collectively comprise Google’s “satellite” views. As you zoom out, the view switches from air photo to satellite image, with each selected to emphasize present-day features.
One idiosyncrasy of Google’s satellite views at our latitude is that they always display images obtained around noon in early summer. At that time the sun is high overhead, shadows are at a minimum, illumination is even, and vegetation records recent vibrant spring growth. What is shown is a pleasant midday summer view, selected without a cloud to be seen. I wish that weather scenario was always the case.
However, you can purchase your own images from TerraMetrics for any season over the past few decades. Two satellite systems – Copernicus and Landsat – provide much of that imagery. Copernicus is European sponsored, while Landsat is a joint NASA/USGS mission with satellites that have been continually scanning Earth since 1972. The land observation satellites of both missions orbit from the north to south pole and back again, constantly imaging the daytime surface as Earth rotates beneath them. They capture images of the same location about every two weeks. That means TerraMetrics has many image dates to select from.
More significantly, this and other satellite data over the past 50-plus years provide the most comprehensive record of global environmental change that is available. Changing vegetation, land use, cloud and snow cover are readily measured. Some of these changes can be reviewed at eros.usgs.gov/earthshots and earthobservatory.nasa.gov/global-maps.
The town of Orange contracts with an imagery broker that can highlight your own or other properties at hosting.tighebond.com/orangect_public/. Milford’s is at https://milford.mapxpress.net/. Use is free, and once you type in an Orange property street address and click on the “layers” tab, you are able to view a range of map overlays with property boundaries, including Google satellite views as well as detailed aerial photographs of Orange from different years dating back to 2012.
The aerial photos are wintertime views, and a bit gloomy with pronounced shadows given the low sun angle. But there are no leaves on trees to hide views of buildings, pavement surfaces, fences, property lines, rooftops, parked vehicles and other human artifacts. These photos track features and changes that help revalue property and related insurance risks. Note that the photos selected by the office of a tax assessor or insurance underwriter, perhaps anticipating homeowner remorse when their bills arrive, are always of winter.
Dan May is a local geologist.