Considering the vintage, or date, when satellite or aerial imagery was acquired is essential information in investigating land use, vegetation health, or any other variable using GIS and Remote Sensing technologies. In the pre-cloud days, this information was typically included in the metadata that accompanied the imagery files. Nowadays, with an increasing amount of imagery streamed from web-based mapping services, the imagery may be tiled from a multitude of sources, dates, and resolutions, making it a challenge oftentimes to find the date of a specific image that one is examining. The image may have a general statement about the image sources and dates for the globe, but may be lacking metadata for each image. One of the image services in one of these web-based mapping platforms, ArcGIS Online, makes it easy to find the dates and resolution for the imagery presented, and provides a good example that other image services could follow.
This image service shown on the map at this URL provides one meter or better satellite and aerial imagery in many parts of the world and lower resolution satellite imagery worldwide. The map includes NASA Blue Marble: Next Generation 500m resolution imagery at small scales (above 1:1,000,000), i-cubed 15m eSAT imagery at medium-to-large scales (down to 1:70,000) for the world, and USGS 15 m Landsat imagery for Antarctica. The map features 0.3m resolution imagery in the continental United States and parts of Western Europe from DigitalGlobe. Additional DigitalGlobe sub-meter imagery is featured in many parts of the world, with concentrations in South America, Eastern Europe, India, Japan, the Middle East, and Northern Africa, Southern Africa, Australia, and New Zealand. In other parts of the world, 1 meter resolution imagery is available from GeoEye IKONOS, Getmapping, AeroGRID, IGN Spain, and IGP Portugal. Additionally, imagery at different resolutions has been contributed by the GIS User Community.
This map includes metadata containing the date the image was acquired as well as the source image resolution. Simply click on the map to view this metadata, as I have done below, showing that the image on the west side of Denver, Colorado, USA, was acquired on 16 March 2012 at a resolution of 0.30 meters.
Imagery vintage in Denver, Colorado USA
The base imagery in ArcGIS Online is continually updated and as the above description shows, is a mosaic of a variety of images. Being able to access the specific date of acquisition for specific images is of great value to the GIS user. Moreover, this single map can be used to investigate changes on the landscape due to the time of day, the time of year, natural changes to the landscape, and changes to the landscape made by humans. For example, search and zoom to the southwest side of Brandon, Manitoba, Canada, around latitude 49.837892 North and longitude 100.003871 West. You can see on that map, shown below, a place where four different images have been stitched together, covering a span of 9 years. A closer examination reveals urban growth as well as changes from summer to winter.
Images of different vintage spanning 9 years over Brandon, Manitoba, Canada.
Because image services such as this are frequently updated, Brandon likely won’t look this way for long. But there will be other locations where differing vintage in mosaicked image tiles is evident. As we state in our book, make sure you understand the data you are using, including the spatial and spectral resolution of the images you are examining, along with other key data such as the vintage of the imagery.
Tagged: data quality, Metadata, on-line mapping, satellite imagery
