Airborne platforms usually offer higher temporal resolution than orbiting space-borne platforms because orbiting platforms are restricted by their orbits, which determine how often and when a spaceborne platform will pass over a specific location on the earth. Conversely, airborne platforms can be flown at any time of day or night but may be restricted from flying over a specific area such as a war zone.
Because passive systems rely on the energy of the sun, they will always have lower temporal resolution than active systems, which can be flown at any time of the day or night. Because satellites are tied to their orbits, they will never have the temporal resolution of aircraft systems in countries where airspace is relatively unrestricted (as opposed to severely restricted airspace over countries such as Iraq or North Korea). Sun-synchronous satellites capture imagery during the same period every day, which reduces their versatility. Aircraft can often fly under clouds, and image collections can be specifically timed to tidal stages, crop calendars, or deciduous leaf conditions.
Extent
The term extent is used to refer to the area on the ground that can be captured with each exposure of the sensor. It is often used relative to one mission or collection and is determined by the sensor size, its focal length, and its distance to ground. Many satellite systems collect imagery continually along their orbital paths. The area collected is, therefore, constrained by the width of the sensor’s swath, but not by the strip length. The length of individual satellite scenes is arbitrary and is determined by the operator of the system. Most scenes are approximately square. For example, a Landsat scene is 170 by 183 kilometers.
The size and shape of a project area will affect what remote sensing systems are most suitable for imaging it. Because of their altitude, satellites can capture large areas in individual satellite scenes (e.g., a Landsat scene covers just over 12,000 square miles). However, satellite images are restricted to the satellite’s orbital paths, making aircraft systems more effective in collecting linear or sinewy project areas (such as riparian areas, transmission lines, and coastlines) because aircraft are not tied to an orbit.
Helicopters are ideally suited to collect data over multiple distributed points because they are more agile than airplanes. They are also suitable for collecting data along corridors where frequent turning may be necessary. Fixed-wing aircraft are more suitable for collecting imagery over large areas because they can quickly collect large swaths of data. For extremely large areas, a high-altitude aircraft or satellites might be employed to maximize ground coverage.
For example, a riparian mapping project, following a long sinewy river with a required spatial resolution of 1 meter will probably be better accomplished with an airborne system than with a satellite system. The airborne platform can follow the path of the river and constrain its data collection to only the river area. Using data from a satellite system would require collecting multiple scenes, and then extracting the river areas from the larger scenes. More area and thus more data than required would be collected, which would increase the cost of the project.
An advantage that moderate-spatial-resolution, large extent satellite systems (e.g., Landsat and Sentinel) have over airborne collections is that the entire scene is captured at once with instantaneous sun illumination, vegetative condition, and atmospheric conditions fixed across the scene. Capturing the same area as a Landsat scene with an aircraft system would take several days, with the sun illumination, vegetation, and weather conditions changing throughout each day and from day to day. These variations can introduce confusion when the images are manually interpreted or classified to create maps.
High-spatial-resolution systems (both airborne and satellite) have a smaller extent than moderate- and low-spatial-resolution systems, but individual scenes can be mosaicked together to represent a larger area. Mosaicking is discussed in more detail in chapter 5.
Organizational Characteristics
Introduction
The choice of what imagery best meets a project’s requirements will be determined not only by the imagery collection characteristics but also by the imagery’s organizational characteristics. Organizational characteristics are determined by the organization(s) funding the imagery acquisition and distribution. Types of organizations include public agencies, private companies, and organizations with a combination of public and private funding. Organizational characteristics affect the imagery’s accessibility and price to users. This section introduces and reviews imagery organizational characteristics—its pricing and licensing and its accessibility.
Pricing and Licensing
Price is the amount a user pays to gain access to imagery. Licensing refers to the restrictions placed on the use of the imagery. Licensing and pricing are often linked. Much of the medium- and low-spatial-resolution remote sensing data collected is free and its use is unrestricted (i.e., the data is in the public domain). Other imagery, especially high-resolution satellite imagery, is either severely restricted by government policy or accessible only through the purchase of a license with associated restrictions on the user’s ability to share the imagery with other users.
Because the primary demand for low- and moderate-spatial-resolution imagery is from the public sector, acquisition and distribution of much of the low- and moderate-spatial-resolution civilian satellite imagery acquired by the US government and the European Space Agency (ESA) is funded by taxpayers and available to most users at no charge with few or no use restrictions. As a result, NASA earth observation data, Landsat imagery, National Oceanic and Atmospheric Administration (NOAA) weather imagery, and the ESA’s Sentinel imagery are all freely available to most users worldwide.
The collection of high-resolution airborne imagery in the United States is usually purchased as a service by government agencies from commercial providers. The agencies pay the provider to collect and process the imagery, with the agency retaining all or most rights to the imagery. Similar to the availability of low- and moderate-resolution satellite imagery, most high-resolution airborne imagery is made available by agencies to the public at no cost, although some charge user fees. The USDA NAIP collects high-resolution, 4-band multispectral imagery over one-third of the continental United States every year at 1-meter spatial resolution. The imagery is available to the public at no cost and with no user restrictions. Most of the NAIP commercial providers also offer the ability to “upgrade” the imagery to 30- centimeter spatial resolution on a paid subscription basis. The upgrades are available because the providers capture the imagery at the higher resolution and resample it to the lower resolution for the public domain product. The provider then makes the higher resolution product available through a licensing agreement that restricts the use of the imagery (i.e., the purchaser is restricted in some way from copying or sharing the imagery with other organizations). Besides NAIP, many states and local governments retain commercial firms to collect airborne high-resolution multispectral and lidar data over their jurisdictions. Usually, the imagery is made available to the public at low or no cost, and with few, if any, use restrictions. Private companies such as utility and forestry firms also contract with airborne providers to produce high-resolution imagery of their properties.
Until recently, high-spatial-resolution satellite imagery was either completely government funded with use severely restricted, or partly government funded, with use restricted by licensing.