Surface Reflectance Basemaps

last updated: November 19, 2021

Planet’s Surface Reflectance Basemaps are 16-bit, time series mosaic products which are optimized for radiometric consistency and minimize the effects of clouds, haze, atmospheric effects, and other image variability. They are ideal for use in forestry, vegetation, and land cover mapping use cases to enable an understanding of change over time.

PlanetScope Surface Reflectance Basemaps (Zoom Level 15 - 4.77 meter, and Zoom Level 16 - 2.38 meter cell size at the equator) are generated with a proprietary algorithm which harmonizes the source imagery to Landsat and uses cloud masking to mosaic only the best underlying pixels. Surface Reflectance Basemaps can be purchased over custom areas of interest at a quarterly, monthly, biweekly, or weekly cadence. We do not currently offer Surface Reflectance Basemaps for our SkySat product line.

Surface Reflectance Basemaps are available for download via the Basemaps API and Basemaps Viewer, and can be streamed via Planet Web Tile Services.

You can read a detailed overview of our Basemap Product Specification here.

Imagery Products

Overview

Below is a high level overview of our PlanetScope Surface Reflectance Basemap product.

Source Imagery Download (GeoTIFF) Bands Streaming (WMTS) Bands Monitoring Frequency Zoom Level Normalization
PlanetScope analytic_sr assets BGRN RBG, NRG Quarterly, monthly, biweekly, weekly 15, 16 Landsat, none (non-normalized)

Cadence

Basemaps are generated at a specified cadence based on the “first_acquired” and “last_acquired” UTC timestamps for underlying source imagery.

Cadence first_acquired last_acquired
Quarterly January 1st, April 1st, July 1st, and October 1st at 00:00:00 UTC March 31st, June 30th, September 30th, and December 31st at 23:59:59 UTC
Monthly The first day of each month at 00:00:00 UTC The last day of each month at 23:59:59 UTC
Weekly Mondays at 00:00:00 UTC Sundays at 23:59:59 UTC

Scene Provenance

During the Basemap generation process, a record of each individual PlanetScope or SkySat image used is retained. All source scenes are traceable through Planet’s Basemaps API and Basemaps Viewer.

Basemap Quads

Basemaps can be downloaded as a set of “basemap quads”, or simply “quads.” Quads are a distributed grid of GeoTIFF files which compose the basemap.

Basemap with quad boundaries

An individual Surface Reflectance Basemap quad has the following standard specifications:

Attribute Description
Imagery PlanetScope
Pixel size (resolution) 4.77 meter (Zoom Level 15) and 2.39 meter (Zoom Level 15) at the equator
Image bit depth 16 bits per pixel
Bands Blue, Green, Red, NIR, Alpha
Projection WGS84 Web Mercator (EPSG:3857)
Size 4096 x 4096 pixels

The projection used in Planet Basemaps has been selected to match what standard web mapping applications (Web Mercator Projection). The last band in the GeoTIFF of every mosaic quad includes an Alpha Mask which indicates areas of the quad where there is no imagery data available.

Single quad within a basemap

Product Naming

The name of each PlanetScope Surface Reflectance Basemap is custom and will be made available to you following the purchase of your basemap product.

The name of each basemap quad within the Basemaps API is designed to represent the x and y position of the quad within the two dimensional grid which makes up the basemap. It is generally {X}-{Y}, where X and Y are the x and y position of the quad in the grid.

Example: 439-1220

Upon download, the name of the downloaded quad is designed to represent the zoom level and the mosaic’s tiling scheme.

Example: L15-0439E-1220N.tif

Processing

Normalization

Following scene selection, Surface Reflectance Basemap source scenes are normalized to a quarterly Landsat reference dataset to minimize variability between scenes and improve spatial and radiometric consistency.

Each scene is fitted with a linear model based on co-located, non-cloudy Planetscope and Landsat reference pixels to transform the input reflectance coefficients to approximately match the Landsat reference values for each band.

After adjacent scenes have been combined into a mosaic tile, a seamline-reduction algorithm is applied to minimize any remaining local differences between scenes. This algorithm is optimized for landmass coverage, and may exhibit inconsistencies in visual quality over open water.

While Planet cannot guarantee that a basemap will not contain visible scene lines or artifacts resulting from the mosaicing process, these approaches generally make the imagery appear more consistent and seamless.

Packaging

Basemaps are converted into a Web Mercator projection and resampled to a default pixel size of 4.77 meters. The resulting quads are then indexed within the Planet platform so that they may be downloaded for offline use via the Mosaics API.

Lower zoom level overviews are created to populate the full stack of web tiles. These feed into Planet’s Web Tile Services, which are easily integrated in other applications, serving up only the part of the basemap a user needs.

Quality

In addition to basemap production techniques described above to ensure quality output, basemap quality and coverage are also functions of source imagery input constraints based on the specified time of interest and cadence of delivery.

For a smaller time of interest and shorter cadence periods, basemap quality and coverage are more likely to be impacted.