Time series or temporal imagery creates an opportunity to systematically make comparisons over time. The result is a broad array of potential time-traveling applications such as before-and-after views of cataclysmic events and reconstruction of historical landscapes, as well as monitoring and forecasting change over weeks, months, years, and decades. The study of dynamic processes over time is the big idea of this chapter.
It’s about time
Using imagery to monitor our dynamic planet
To truly understand our dynamic planet, we strive to explore information through time, visualize the past, understand the present, and recognize future trends. For example, earth scientists use a time series of satellite observations to track monthly precipitation patterns such as snowpack coverage and extent as it descends from the higher latitudes and the poles during the winter months and then recedes in summer. Scientists use satellite time series observations to monitor droughts. And climatologists apply models to forecast climate trends for future points in time. As computing and GIS continue to grow, new time-based capabilities are being developed and applied. Along with this trend, there is a growing appreciation for the critical importance of imagery’s temporal aspects. Our world is dynamic so it makes sense that GIS reflects that. And imagery plays a pivotal role.
There are countless applications that require temporal considerations. Delineating a wildfire boundary requires a series of missions to collect thermal imagery for a focused area around the fire perimeter. These are often flown several times a day since wildfires change rapidly. In contrast, exploring deforestation requires imagery covering a large area but for longer time spans, since deforestation is a process that occurs over periods of many years or decades.
Continuous global observation
Meanwhile, there is no lack of raw information coming from sensors to address many of these challenges. There is an ongoing explosion of continuously observing satellite platforms—both government and commercial—all contributing to our collections of earth observations. A number of satellites are designed for continuous observation, revisiting the same areas over repeating time periods—Landsat, MODIS, GLDAS, Sentinel, SPOT, and RapidEye, to name just a few. Landsat 8 revisits every Earth location about every 16 days (its “revisit period”). MODIS collects observations resulting in global coverage every one to two days. And so on.
It’s about time.
You perceive temporal transformations as they happen, such as the passing of fall to winter and the shift from day to night. Imagery mimics and extends human perception to much grander temporal and spatial scales. Time-aware imagery empowers us to ask and answer questions that transcend our personal time and space.
Types of time
Discrete, cyclical, and continuous
Time can be viewed as linear or cyclical. Linear time has a distinct beginning and end and can be expressed using discrete, continuous, or cyclical measures of time. Video is an example of imagery captured in continuous time. Cyclical time captures events that occur in a sequence over and over. Weather that is observed daily is an example of cyclical time.
Historical imagery
Imagery is one of the most effective and moving ways to capture the past. Historical imagery serves as the benchmark for detecting change and allows us to make better decisions in managing the earth’s valuable resources. GIS technology brings new life to historical maps and old photographs—on the ground, from the air, and even via outer space.
Historical scanned maps
Scanning paper maps turns them into imagery. Once scanned, they can be georeferenced and included in a GIS just like any other layer. Historical maps can provide context for your analysis and serve as a basis for change detection. The USGS has one of the world’s largest collections.
Historical aerial photographs
This type of imagery is taken from the air using balloons; aircraft; and more recently, drones. It provides a planimetric (bird’s eye) view of the landscape. The first aerial images were taken in the late nineteenth century.
Historical terrestrial photographs
Photography was the first method available for capturing imagery. Historical photographs can provide context and perspective for a GIS analysis. You can locate the same place photographed a hundred years ago and photograph it again for imagery comparison, to see what’s changed, as the USGS Repeat Photography Project did along the Colorado River.
Modeling Earth’s processes
There are a number of Earth observation satellites that pass over selected areas of the planet daily to produce a time series of imagery. For example, weather satellites show the day’s weather as it happens in near real-time. Meanwhile, a number of satellites such as GLDAS pass over all global areas daily producing critically important observations about our planet.
Interestingly, multidimensional computer simulation models of Earth’s physical processes are used to compute results with a time series of images, often to interpolate what happens in between satellite passes. Other models can also simulate historical conditions to re-create past events as well as forecast future conditions.
Studying the past
Imagery enables us to digitally reconstruct past landscapes. We can use one historical image to study a location in the past or a collection of historical images to see in more detail how something has changed over space and time.
Organizing imagery collections and historical maps through time into mosaics provides an especially useful tool for comparing previous landscapes with each other and with the present.
Water loss in the Aral Sea
Spyglass on Chicago before the Great Fire
Thought Leader: Greg Allord
Historical maps provide a window to the past and context for the future
For many years at the US Geological Survey (USGS), I was able to support, observe, and greatly respect the work of skilled geologists, hydrologists, engineers, water quality specialists, illustrators, editors, and cartographers. Be it the artistry and precision of the cartographers, the era of print media, the interpretative science, or the geographic extent, the content of the maps and publications that our teams created appealed to a wide variety of people.
Organizations studying the environment, sciences, and culture of the United States have been in existence for many decades, some dating as far back as the first years of this country. The USGS has had a 136-year legacy of geologic, topographic, water, and biologic resources studies. It is also where I had the privilege of working. At the USGS, the transition moved us from manual methods, into a period of digital exploration, to now, with the promise of digital cartography and map sharing having been realized.
Librarians have taught us that it is not sufficient to simply house a hard-copy print collection like this; it must be cataloged and preserved. An effective preservation process is capturing an image of the original. Traditional paper maps and reports, in particular, benefit from this. The result: more than 130,000 USGS science publications can be accessed at http://pubs.er.usgs.gov, and approximately 175,000 topographic maps can be accessed at http://nationalmap.gov/historical/index.html or used in ArcGIS Online. Not only does this make it possible for researchers and scientists to use these maps and documents, but it also allows a broader audience to appreciate and experience their beauty and importance.
The historical maps the USGS and other agencies created provide not just a window into yesterday, but also a context for tomorrow. While it is not possible to predict all users and uses of this information, the accessibility of the archive of historical maps through ArcGIS Online and the Living Atlas provides the world with the ability to better understand the past, manage the present, and plan for and share the future.
View the historic map collection
Historical map collections
Historical maps add an important dimension to GIS. They offer a clarity and shape to what our world was like in the past. They speak of the possibilities of places we can never visit again except through the use of these maps. They offer a framework for comparisons between now and the past as well as into the future. One great thing about historic maps is that they can be integrated with maps and information from today’s world. In effect, they can be added as new kinds of layers to your GIS. The way this is done is by scan-digitizing the historic maps and georeferencing them. They essentially become a new kind of raster layer in your GIS and create enormous opportunities for many types of applications.
Landsat is a time machine
First satellites to provide continuous global observation
Landsat sees Earth in a unique way. It takes images of every location in the world to reveal Earth’s secrets, from deforestation patterns, to agricultural trends, to volcanic activity, to urban sprawl. The Landsat program started collection with early sensors in the 1970s and continues with the current Landsat 8 mission. Since every part of the earth is captured every couple of weeks, this enables us to see and analyze how places change over time.
Publicly shared imagery
The USGS manages the Landsat data program and makes the imagery freely available for everyone. This collection has been continuously updated with new scenes from various Landsat sensors for over four decades, resulting in an amazing, historical earth imagery resource.
New Landsat scenes are being collected daily. As new scenes are generated, they are added to a dynamically growing image mosaic containing millions of existing Landsat scenes in the shared database, providing extraordinarily useful information for historical comparisons.
Map, measure, and monitor everything
Since the start of the Landsat program, a number of government agencies worldwide have launched their own missions—MODIS, the European Space Agency’s (ESA) Copernicus program and its recent Sentinel-2 satellite pair, and many more—to continuously gather imagery and publicly share earth observations. New missions are being launched regularly, providing a growing collection of time series earth observations from space: a macroscope for the planet.
Historical map mosaics
Collections are easy to compile
Many maps are singular in nature, while others belong to a large map series or map collection. The USGS topographic map series, flood map series, insurance map series, aerial photos from specific missions of the past, the David Rumsey collection, the storehouse of historic maps from National Geographic—all are examples of map collections that can be used to enrich your GIS.
A useful approach for organizing and providing access to large historical map collections is to generate an image mosaic of your collection. The properties of each map—its name, creation date, spatial reference, and other characteristics—are recorded as attributes and used to create a seamless mosaic dataset. Mosaics help to bring your entire map collection to life in your GIS, enabling numerous applications and uses.
USGS Historical Topographic Map Viewer
The USGS Historical Topographic Map Collection includes all scales and all editions of the more than 175,000 topographic maps published by the USGS since its inception in 1882.
Use the slider to change the transparency of various historic maps. Right-click a map to download and share it with your friends and colleagues.
Two hundred years of Dutch topography
Created and shared by the Dutch Cadastre
The Dutch Cadastre (The Netherlands’ national mapping agency) generated a series of precached maps at numerous map scales (and for each year) for the entire country, stitching together the comprehensive collection of historic topographic maps. It is a magnificent cartographic treasure for the Dutch people and Dutchophiles everywhere.
Event-driven analysis
At the right time and the right place
GIS users seek to understand the results of an event by comparing the most recent situation to a previous state that is days or hours in the past. Common applications of near real-time imagery include coordinating emergency response, performing damage assessments, monitoring forests and agriculture, conducting military operations, and much more.
The term “near real-time” refers to time ranging between just before an activity or event and up to two to three days after the activity or event has occurred. This time range is often described as either real-time or near real-time, depending on how close or far (in temporal terms) you are from the event around which people are responding.
When considering event-driven imagery, it is important to determine how frequently you need to observe conditions affecting the event. During a hurricane, frequent images of the area can help you detect subtle changes in the storm direction and speed. In other words, the ability to be looking at a situation in a specific location can significantly aid the effort to defend life and property. GIS analysts assist in the fight by sampling such changes at a higher frequency, since timely decisions are likely to be more effective.
Disaster preparation and response
Forecasting areas susceptible to wildfire
Weather forecasting in GIS
When meteorologists look at satellite imagery, they see more than just current weather. They peer into future weather as well. Meteorologists can analyze current conditions and forecast what will happen next.
Imagery helps to see into the future
Climate and weather forecasting
For future scenarios like climate change or sea level rise, effective response begins in the present—well in advance of the potential results and impacts that can occur. GIS makes it possible to run complex models into the distant future, enabling us to better understand the potential impacts.
Exploring our dynamic world
There can be no argument that the planet is changing rapidly. People born in the twenty-first century will experience more dynamic change in a shorter period of time than that experienced by at least the last few dozen generations. Imagery and GIS help people understand and share what these impacts could be and how we might mitigate the situation.
Case study: NOAA sea level rise
Our planet’s future sea level depends on decisions that have yet to be made and cannot be predicted now. However, what’s going to happen over the next 100 years is pretty well understood: Sea levels will rise by at least another three feet, even if we stop emitting carbon tomorrow, and by as much as six feet if we continue increasing emissions at our current rate. To help engineers, city managers, and concerned citizens understand what this means for their neighborhoods, NOAA’s Office for Coastal Management has developed a Sea Level Rise Impact Viewer with imagery forecasting what the nation’s coasts will look like as the ocean rises.
This data viewer provides coastal managers, scientists, and citizens with a preliminary concept and understanding of sea level rise and coastal flooding impacts. The viewer is a screening-level tool that uses nationally consistent datasets and analyses. You can interact with the data and maps at several scales to help gauge trends and begin to articulate potential responses for different scenarios. A slider allows you to envision what various locations might look like with one foot, two feet, or up to six feet of sea level rise.
Tabs switch the map from natural color imagery to layers showing land-use classification and socioeconomic vulnerability, providing important context beyond just the visual impact of seeing buildings underwater. The application even forecasts how the land cover will change as sea levels rise, focusing on the impact to wetlands and marshes.
Quickstart
Get inspired by these examples of real-world problems explored through temporal imagery
Learn ArcGIS Lesson
Depict land-use change with time-enabled apps
Over the last 40 years, Thailand has experienced tremendous shifts in land use due to a booming aquaculture industry. Widespread flooding of land to create shrimp farms has impacted sensitive ecosystems across the country, but particularly along the coast. As a GIS analyst for a nonprofit organization focusing on conservation and sustainable land practices, your goal is to find historical Landsat imagery for the Samut Songkhram province south of Bangkok in order to create a visual report of how the environment has changed over time. Your presentation will be shown to donors and investors to procure funding and promote the restoration of coastal ecosystems.
In this lesson, you’ll build a presentation for identifying which region of Samut Songkhram province should be the focus of conservation efforts. You’ll retrieve one image per decade since the 1970s from the Living Atlas Landsat archive for the entire study area. Once you have the images, you will analyze the available multispectral data to enhance vegetation, land, and water. Then, you will configure the time-animation tool in ArcGIS Online, and create custom time-aware apps for publishing your observations.
Build skills in these areas:
- Adding Landsat imagery to a map
- Enabling and configuring time animation
- Filtering historical satellite imagery
- Changing band combinations
- Creating and sharing time-enabled web apps
- Publisher or administrator role in an ArcGIS Online organization
- Estimated time: 40 minutes
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Creating Mirror Worlds
Enabling a new dimension
