Glossary

Alpha Mask

An alpha mask is an image channel with binary values that can be used to render areas of the image product transparent where no data is available.

Analytic

An alpha mask is provided as a color channel in scene products to mask out corrupt or low signal-to-noise ratio pixels. In the case of older PlanetScope 0 and PlanetScope 1 imagery, this mask gives a circular profile due to vignetting. Customers may remove the alpha mask.

Analytic

The analytic Scene product is 12-bit data delivered in a orthorectified 16-bit GeoTIFF file with no visual color-corrections applied; it is designed for analytical and scientific applications.

Analytic XML

The analytic XML file contains a description of basic elements of the Analytic asset. The file is written in Geographic Markup Language (GML) version 3.1.1 and follows the application schema defined in the Open Geospatial Consortium (OGC) Best Practices document for Optical Earth Observation products version 0.9.3.

Area of Interest (AOI)

An area of the world that is of interest for finding imagery.

Application Programming Interface (API)

A set of routines, protocols, and tools for building software applications.

Atmospheric Correction

The process of removing the effects of the atmosphere on the reflectance values of images taken by satellite sensors. Currently Planet does not provide atmospheric correction.

Bayer Mask Color Filter Array

The Bayer mask filters light reaching the CCD. At the time of image capture, each CCD photosensor measures one of the Red, Green, or Blue wavelengths. These values are interpolated to assign a red, green, and blue value to each image pixel.

Blackfill

Non-imaged pixels or pixels outside of the buffered area of interest that are set to black. They may appear as pixels with a value of “0” or as “noData” depending on the viewing software.

Blooming

Blooming occurs when the electrons that collect in the CCD well buckets (during TDI) overflow the bucket in the brightest part of the image. The structure of the CCD allows the electrons to flow more easily in one direction than another, meaning that blooming manifests itself as vertical streaking in the image.

Charged-Coupled Device (CCD) Sensor

CCD sensors use capacitors to represent pixels. Each capacitor allows for the conversion of incoming photons into a digital number. CCD’s are commonly used for the acquisition of professional, medical, and scientific imagery.

Digital Elevation Model (DEM)

A digital model of the elevation of the Earth’s terrain surface relative to an ellipsoid. A DEM is used to remove terrain distortions from the imagery for the geo-corrected products.

Digital Number (DN)

The value assigned to a pixel in a digital image. This gray density value represents the intensity of reflected light from a feature collected by the sensor for a particular spectral range.

Dynamic Range

The number of possible DN values for each pixel in a band of an image.

Ellipsoid

A standardized three-dimensional geometric model of the distance from the earth’s center point to the earth’s surface that is used to establish a common baseline for the measurement of local variation in the elevation of terrain

Field of View (FOV)

The 2 dimensional (X,Y) area on the ground visible to a single Planet satellite.

Flat-Field Correction

All Planet scenes are flat-field corrected, which undoes the effects of vignetting, as well as normalized for sun angle and camera settings. This happens alongside radiometric correction and is considered the same process.

Frame ID

The sequential ID for individual scenes relative to the frame camera that captured them. SkySat spacecraft collect imagery in strips (strip_id) using three overlapping frame cameras. During imaging, each of these cameras (d1, d2, d3) captures its own strip of scenes, which are indexed sequentially using the Frame ID.

Gain Optimization

Gain is a measure of the electrical amplification of the signal created by the CCD sensor when a photon interacts with it. TDI allows for the optimization of gain settings; gain optimization allows Planet to maximize an image’s dynamic range, or the maximum and minimum light intensities.

GeoJSON

A standard for encoding geospatial data using JSON (see JSON below).

GeoTIFF

Georeferenced tagged image file format. A GeoTIFF file is a version of the TIFF file format that contains embedded geographic positioning data. GeoTiff is a commonly used image file format used to associates pixels with geographic locations.

Global Land Area (GLA)

Used to estimated area coverage capacity, where one GLA is approximately 150 million square kilometers.

Ground Control Point (GCP)

A point on the ground with known geographic coordinates. GCPs can be planimetric (latitude, longitude) or vertical (latitude, longitude, elevation). GCPs can be collected from ground survey, maps, or orthorectified imagery.

Ground Resolved Distance (GRD)

The dimensions of the smallest object resolved in an image.

Ground Sample Distance (GSD)

The distance between pixel centers, as measured on the ground. It is mathematically calculated based on optical characteristics of the telescope, the altitude of the satellite, and the size and shape of the CCD sensor.

Graphical User Interface (GUI)

The web-based graphical user interfaces allows users to browse, preview and download Planet’s imagery products.

Histogram Matching

An algorithm that matches the cumulative distribution functions of the red, green, and blue spectral bands between two images to balance differences in image illumination or sensor calibration from one image to another. The code that makes our version of histogram matching is open sourced and available on GitHub.

International Space Station (ISS) Orbit

International Space Station (ISS) orbits at a 51.6°inclination at approximately 400 km altitude. Planet deploys satellites from the ISS, each having a similar orbit.

JSON (JavaScript Object Notation)

Text-based data interchange format used by the Planet API.

JPEG

Lossy compression technique for color images that reduce files sizes to about 5% of their normal size. JPEGs and PNGs are the available thumbnail file formats for Planet products.

Landsat 8

A satellite operated by the US Geological Survey (USGS) that was launched by the National Aeronautics and Space Administration (NASA) in February 2013. Landsat is history’s longest-running Earth imaging project. Landsat 8 scenes are available through the Planet API.

Latency

The amount of time that passes from the moment an image is captured to the moment the image is available in the APIs.

Lempel–Ziv–Welch (LZW)

Lempel–Ziv–Welch compression. A lossless compression algorithm which retains all image information with a modest reduction in file size.

Metadata

Data delivered with Planet’s imagery products that describes the products content and context and can be used to conduct analysis or further processing.

Mosaic

A composite of many individual images into a single layer.

Mosaic API

Primary access mechanism for downloading mosaics. The Mosaics API offers REST API access for listing mosaics, listing and filtering quads, downloading quads, and listing available scenes used as input to each GeoTIFF quad.

Mosaic Explorer

Web-based graphical user interface (GUI) that enables access to mosaic data.

Multispectral

Imagery recorded in multiple discrete spectral bands. Multispectral usually refers to imagery collected in 3-10 bands or wavelengths of the electromagnetic spectrum.

Nadir

The point on the ground directly below the satellite.

Near-Infrared (NIR)

NIR is a region of the electromagnetic spectrum that is invisible to the human eye and of particular value in remote sensing of vegetation. Planet is actively developing NIR imaging capability.

Off-Nadir Angle

The angle between the nadir point and the actual point on the ground where the satellite is pointed.

Orthorectification

The process of removing and correcting geometric image distortions introduced by satellite collection geometry, pointing error, and terrain variability. Rectification also includes re-sampling the imagery to a specific map projection. All orthorectified scenes are referenced to the geographic registration of existing Landsat 8 panchromatic imagery (or better) and are orthorectified with the SRTM digital elevation model. Our Analytic and Visual products are orthorectified.

Ortho Tile

Ortho Tiles are Planet’s core product lines of high-resolution satellite images. Ortho tiles are available in two different product formats: Visual and Analytic , each offered in GeoTIFF format.

Panchromatic

A single wide spectral band which combines the reflected light in all wavelengths of the visible spectrum (blue, green, red) and sometimes NIR. Since panchromatic images capture more photon energy (per unit of area on the sensor), they are typically used to produce higher spatial resolution images than R,G,B or multi-spectral images made with comparable optics or sensors. Panchromatic images are typically displayed in grayscale.

Pansharpened

A multispectral image that has been fused with a higher spatial resolution panchromatic image to create a color image with improved resolvability.

Pixel Compositing

At Planet, mosaics are built by automatically merging many scenes, picking the best source pixel for each location on the earth. The compositing algorithm attempts to select the best pixel for each location based on a set of optimizing criteria including absence of clouds and cloud shadows, source image resolution, temporal appropriateness, and other image quality characteristics. The code that makes this advanced pixel level compositing possible is available in the planetlabs/plcompositor repo in Github.

PlanetScope

The first three generations of Planet optical systems are referred to as PlanetScope 0, PlanetScope 1, and PlanetScope 2.

PNG

A raster graphics file format that supports lossless data compression. PNG and JPEG are available thumbnail file formats for Planet images.

Positional Accuracy (Geolocation Accuracy)

The difference in geolocation of Planet image pixels compared to some external basemap. Positional accuracy is frequently quoted in RMSE (root-mean-square error) and CE90 (circular error at 90% confidence).

Quad

Each Planet mosaic is served as a number of GeoTIFF quads that, together, compose the complete mosaic.

Radiometric Correction

The correction of variations in data that are not caused by the object or image being scanned. These include correction for relative radiometric response between detectors, filling non-responsive detectors and scanner inconsistencies.

Radiometric Normalization

The process of normalizing images across sensors and time in order to support change detection and mosaicking use cases.

RapidEye

RapidEye refers to the five-satellite constellation in operation since 2009.

Raster

A raster is a data structure used to represent a contiguous array of data (pixels); Our images are continuous raster products (as opposed to vector data structured products).

Resolution

The resampled image pixel size derived from the GSD.

Revisit Time

The time interval between successive opportunities to image the same point on the Earth’s surface.

Rational Polynomial Coefficient (RPC)

RPCs provide a generic geolocation model which, in combination with an elevation datasource, can be used to georefrence or ortho-correct an Unrectified scene.

Scene

A “scene” refers to a single image as taken by one of the PlanetScope satellites.

Sensor Correction

The correction of variations in the data that are caused by sensor geometry, attitude and ephemeris.

Signal-to-Noise Ratio (SNR)

Signal-to-Noise Ratio or SNR is the ratio between the number of photons received divided by the number of noise photons from the source and electronics on board.

Sentinel-2 Copernicus Sentinel-2 is a multispectral imaging satellite constellation operated by the European Space Agency.

SkySat

SkySat refers to the constellation of high-resolution imaging satellites in operation since 2013.

Stack Depth

The total area that an overlapping number of images share in an AOI. Units represent cloud free area collect capacity and are expressed in GLAs.

Strip

The set of images collected by a single satellite on a single orbital pass.

Sun Azimuth

The angle of the sun as seen by an observer located at the target point, as measured in a clockwise direction from the North.

Sun Elevation

The angle of the sun above the horizon.

Sun Synchronous Orbit (SSO)

An orbit that rotates around the earth at the same rate as the earth rotates on its axis. A satellite in an SSO orbit will image the ground at the same solar time every day, because the orbital path is deliberately synchronized to maintain a constant relative position of the sun to the earth’s surface.

SuperResolution (SuperRes)

The process of resampling a series of input images to create a single output with better resolution and signal-to-noise ratio than the inputs.

Swath Width

The width of the ground area as recorded by one imaging acquisition.

Time Delayed Integration (TDI)

TDI is an image capture technique that uses multiple stages of exposure combined into a single composite exposure.

Thumbnail

Reduced-size versions of image products. Planet offers thumbnails in PNG and JPEG formats.

TIFF

A computer file format for storing raster graphics images. See GeoTIFF.

Web Tile

Web tile: a 256 x 256 pixel PNG format file.

Web Tile Server

A different distribution mechanism for mosaics, optimized for the web. Each “tile” is 256 by 256 pixels, and has different “zoom” levels. At zoom 0, the entire Earth is shown in one 256 pixel square tile. At zoom 1, the Earth is split across 4 tiles, etc. With tiles, a user can easily incorporate Planet imagery as a basemap in an online map or other application.

Unusable Data Mask (UDM)

An UDM provides information on areas of unusable data within an image (eg. cloud and non-imaged data). The bits in the UDM identify whether each pixel contains usable data, blackfill, cloud or anomalous pixels. This mask can be used to determine whether certain pixels in the tile should be used in further analysis.

Unrectified Scene

A scene in the original image geometry, but with various radiometric correction applied. Radiometrically similar to the Analytic product, but without orthorectification. An RPC is provided with this product to assist the user in geolocating or orthorectifying the scene. This is the closest to “Raw” product made available to customers.

Vector

Vectors are a spatial data structure that stores point location and attribute information. Vector representation of spatial information can take the form of points, lines or polygons.

Visual

The Visual product is an orthorectified 8-bit GeoTIFF that has had a standard color curve applied in order to be readily usable in visual analysis and display applications. Related: See Analytic.

Visual XML

The visual XML file contains a description of basic elements of the Visual asset. The file is written in Geographic Markup Language (GML) version 3.1.1 and follows the application schema defined in the Open Geospatial Consortium (OGC) Best Practices document for Optical Earth Observation products version 0.9.3.