Acquiring Imagery: Scanned Maps and Satellite Data

Last updated on 2026-10-17 | Edit this page

Overview

Questions

  • Where can I find historic maps and satellite imagery?
  • What is the difference between a scanned map and a georeferenced image?
  • How do I find and download declassified CORONA and Sentinel-2 imagery?

Objectives

  • Explain spatial, spectral and temporal resolution
  • Download a scanned historic map from a library collection
  • Search for and download CORONA imagery from the CAST atlas and USGS EarthExplorer
  • Search for and download Sentinel-2 imagery from the Copernicus Browser
  • Open a downloaded image in QGIS

Introduction

Paper maps, air photos and other images scanned at high resolution are often georeferenced (given real coordinates) so they can sit under other layers in a GIS. Features in them, such as old buildings, roads or field boundaries, can then be digitized into vector layers for analysis. Satellite images add a view of the whole landscape, often repeated over time.

Many websites offer this kind of raster data for free:

  • Scanned historic maps are usually plain images with no coordinates. You must georeference them before use. (See the section on georeferencing in the Data Management module.)
  • Modern satellite imagery is already georeferenced and can be opened directly.
  • Some sources are offered as web services that QGIS can display without downloading anything.

Resolution: three kinds

Kind Question it answers Example
Spatial resolution How large is one pixel on the ground? Sentinel-2: 10 m; Landsat color bands: 30 m; best CORONA: about 2 m (6 feet)
Spectral resolution How many wavelength bands, and which? A photograph has 3 (red, green, blue); Sentinel-2 has 13, including near-infrared
Temporal resolution How often is the same place imaged? Sentinel-2: about every 5 days; CORONA: occasional missions from 1960 to 1972

Higher spatial resolution shows smaller objects; more spectral bands help tell vegetation, water, soil and buildings apart; frequent revisits let you track change.

Sources

Part 1: Download a historic map (beginner)

The University of Texas Perry-Castañeda Library (PCL) Map Collection hosts thousands of scanned paper maps, organized by region and country.

  1. Go to https://maps.lib.utexas.edu/maps/.
  2. Open the Historical Maps section (or browse a region).
  3. Browse until you find a useful map. Large series, such as military topographic maps, often have index maps that help you find the sheet for a particular area.
  4. To save a map, right-click it and choose Save Image As, and save it in your project folder.
  5. Note where the map came from: title, date, publisher and the page address. You will need these to cite it.

The saved file is an ordinary image. To use it with other layers, you must georeference it in QGIS (Layer > Georeferencer) by matching points on the map to known coordinates.

Part 2: Declassified CORONA imagery (intermediate)

During the Cold War, the United States ran a series of secret photo reconnaissance satellites. The CORONA program (1960 to 1972) produced more than 860,000 black-and-white images, the first satellite photographs of much of the Earth’s surface. The images were declassified in 1995. The best of them, from the KH-4B cameras (1967 to 1972), show objects as small as about 6 feet (2 m), and were taken as overlapping stereo pairs on long, narrow film strips, each covering a strip of ground well over 100 km long.

CORONA images have become a valuable record of landscapes before modern urban growth, dam building and industrial farming. Archaeologists, historians and geographers use them to find ancient sites, old field systems, and settlements that have since disappeared.

2a. Georeferenced CORONA images from CAST

The CORONA Atlas & Referencing System, run by the University of Arkansas Center for Advanced Spatial Technologies (CAST), offers CORONA images that have already been georeferenced, free of charge. It does not cover the whole world, but it is a good first place to look.

  1. Go to https://corona.cast.uark.edu/ and click Explore Atlas.
  2. Red outlines show the areas covered by CORONA missions in the atlas. Zoom in to one of them.
  3. Use the slider at the bottom of the screen to compare CORONA with modern imagery side by side.
  4. Expand the Corona Imagery layer with the + button to see the images being displayed. As you zoom in, the list gets shorter.
  5. Turn images on and off to choose the one you want, then click the blue down-arrow next to it to download it. The files are large, so this can take a while.
  6. Move the file from your Downloads folder to a project folder, for example GIS/CORONA.
  7. In QGIS, go to Layer > Add Layer > Add Raster Layer, browse to the file and click Add.

2b. Raw CORONA scans from USGS EarthExplorer

USGS EarthExplorer holds the complete CORONA collection. Its images are not georeferenced, and the interface takes some getting used to, but it covers far more of the world.

  1. Log in, or create a free account, at https://earthexplorer.usgs.gov/.
  2. In the map on the right, navigate to your area of interest. Either click to draw a polygon around it, or zoom to it and press Use Map in the Search Criteria tab.
  3. In the Data Sets tab, expand Declassified Data and check Declass 1 (1996).
  4. In the Additional Criteria tab, set Camera Resolution to Stereo High. This keeps only the high-quality KH-4B images.
  5. Optionally, set Download Availability to Yes to show only images that are already scanned. Unscanned frames can be scanned on request for a fee ($30 per frame at the time of writing).
  6. Click Results.
  7. Use the Footprint and Image Overlay icons to see where each image falls. The Show Metadata and Browse icon opens a preview, where you can check for clouds and clarity.
  8. When you have chosen an image, click the Download Options icon and download it.

The download is a compressed archive (.tgz). Extract it (on Windows, with a tool such as 7-Zip, you may need to extract twice: first the .tgz, then the .tar inside it). Because each film strip is so long, the image is usually split into several .tif files ending in a, b, c and d. You will need to georeference them before use.

Part 3: Sentinel-2 multispectral imagery (intermediate)

Sentinel-2 is part of the European Union’s Copernicus Earth observation program. Since 2015 its satellites (Sentinel-2A and 2B, joined by 2C, launched in 2024) have imaged all land surfaces about every 5 days in 13 spectral bands, at 10 m, 20 m and 60 m resolution. They are used to map land cover, crops, forests, water, urban growth and disaster damage. The data is free.

Compared with the 30 m color bands of Landsat, Sentinel-2’s 10 m bands show much more detail. Commercial imagery (such as that in Google Earth) can be sharper still, but Sentinel-2’s extra bands, especially near-infrared, and its frequent revisits make it well suited to studying change over time.

Find and preview imagery

  1. Open the Copernicus Browser and log in, or create a free account with the Register button.
  2. Zoom the map to your area of interest.
  3. The panel on the left has two tabs, Visualize and Search. In Visualize, choose a date (days with imagery are highlighted in the calendar) or click Show latest date, and limit cloud cover with the slider.
  4. Choose a layer, such as True color, False color (vegetation shows red) or NDVI (a vegetation index), to view different band combinations.

Search and download

  1. Open the Search tab. Choose Sentinel-2, then MSI, then L2A (surface reflectance, already corrected for the atmosphere).
  2. Set a time range and click Search. Each result is shown as an outline on the map; hovering over a result highlights it.
  3. Click the i button on a result to see its details, including cloud cover.
  4. To download, click the download icon next to the result. The file size is shown, and large products can take a while.

The download is a .SAFE.zip file. Unzip it; the individual bands are stored as .jp2 image files inside the GRANULE/.../IMG_DATA folders, and QGIS can open them with Add Raster Layer.

Callout

Too big to download?

A single Sentinel-2 product is often 500 MB to 1 GB. Download only what you need: in the Copernicus Browser you can also download a single band file, or export just the visible area as an image. The Data Analysis module’s NDVI notebooks show how to work with satellite bands in Python.

Challenge

Find a historic map

Using the PCL Map Collection or Old Maps Online, find a map of your hometown (or of Lafayette, Indiana) made before 1950. Note its title, date, scale and publisher.

Answers will vary. A good record of a map includes its title, date, scale (for example 1:24,000), publisher, the collection that holds it, and the web address. Old Maps Online lets you filter by date with a slider, and shows each map’s footprint on a modern map.

Challenge

Download a CORONA image

Find a CORONA image of your area of interest, or of a famous landmark, in the CAST atlas. If the atlas does not cover it, search EarthExplorer instead. Then open the image in QGIS.

The CAST atlas does not cover every area; if yours is missing, EarthExplorer holds the complete collection. An image from CAST lines up with other layers immediately; an image from EarthExplorer appears without coordinates until you georeference it.

Challenge

Which source?

Which imagery source would you use for each task?

  1. Comparing farmland around a Middle Eastern village in 1968 and today
  2. Mapping where crops were stressed by drought in Indiana last summer
  3. Showing what downtown Lafayette looked like in 1900
  1. CORONA for 1968 (try CAST first), and Sentinel-2 or modern high-resolution imagery for today
  2. Sentinel-2: frequent revisits and a near-infrared band for vegetation indices such as NDVI
  3. A scanned historic map, such as a Sanborn fire insurance map, from a library collection
Key Points
  • Raster data includes scanned maps (which need georeferencing) and satellite images (already georeferenced)
  • Consider spatial, spectral and temporal resolution when choosing imagery
  • Declassified CORONA images (1960 to 1972) are a free historical record; CAST offers georeferenced copies
  • Sentinel-2 provides free 10 m multispectral imagery about every 5 days, from the Copernicus Browser