From Paper to Pixels- Rediscovering Historical USGS Topo Maps in the Google Earth Engine Community Catalog
Maps have long been essential tools for understanding and navigating the world around us. One prominent series of maps that have played a crucial role in the United States is the USGS (United States Geological Survey) Topo Maps. These maps have a rich history, dating back to the late 19th century, and have evolved into a powerful resource for various applications. Among the various scales used, the 1:24,000 scale stands out for its significance in striking a balance between detail and coverage.
The history of USGS Topo Maps traces back to the late 19th century when the USGS embarked on a mission to map the entire United States in intricate detail. The 1:24,000 scale, also known as 7.5-minute quadrangle maps, emerged as one of the most widely used scales. Each map sheet covers an area of 7.5 minutes of longitude and latitude, resulting in a detailed representation of approximately 64 square miles (166 square kilometers). These maps became indispensable for a myriad of activities, including scientific research, land use planning, recreational exploration, and more.
Getting to the Data
The USGS aimed at preserving and utilizing the historical USGS Topo Maps had started to scan and georeference thousands of these maps over varying scales across the US. Knowing the significance of the 1:24000 scale, I started to download over 81,000 maps spanning from the 1900s to the present. The historical USGS Topo Maps at the 1:24,000 scale offers more than just topographic details.
Occasionally, they include ortho photomosaics, providing an additional layer of information. These photomosaics are aerial photographs that have been orthorectified, meaning they have been corrected for distortions caused by the Earth’s curvature and terrain variations. Incorporating such imagery further enhances the understanding of historical landscapes. Alongside the map scans, a 1:24,000 map index or grid was obtained, guiding users to the precise location of each map.
Preprocessing and removing map collars
Ingesting these thousands of maps, totaling over 6.86 terabytes of data, into the Google Earth Engine collection was challenging. However, the effort was well worth it, as this vast archive of historical maps now offers researchers, historians, and enthusiasts an unprecedented glimpse into the past. From understanding how the landscape has evolved over time to comparing historical and contemporary features, the possibilities for exploration and visualization are boundless.
A key challenge arose when stitching together the individual map sheets due to the presence of collars. These map collars contain important metadata, such as map legends and other details, but can obstruct the seamless joining of adjacent maps.
To address this, a function was developed to utilize the 1:24,000 map index layer. By clipping out the collars, the maps could be precisely aligned, creating a unified and comprehensive representation of the region. I further added metadata so there is region and location information available in these map collection metadata apart from year and state which could be used for aggregation.
Dataset availability
The collection of historical USGS Topo Maps with a 1:24,000 scale was part of the insiders’ list and is now generally available in the upcoming release 1.0.9 and will have continued tweaks and improvements. You can find details and the links here. This endeavor was not only an ambitious experiment in unearthing and preserving historical data but also an opportunity to learn about traditional map-making techniques and the art of cartography. Some states are not included yet like California and Texas among a few other states owing to the number of assets and the size limitations.
Try an example script here
The USGS Topo Maps with a 1:24,000 scale provide a fascinating window into the past, offering valuable insights into the changing landscapes of the United States. The effort to digitize, ingest, and stitch together this treasure trove of over 81,000 maps was challenging, but the result is a comprehensive and invaluable resource for researchers, educators, and map enthusiasts alike. Through this project, I can extend my love for historical maps to others and also celebrate the evolution of map-making methods and the timeless importance of cartography in comprehending our world.
