3. Spatial resolution
VHR imagery is characterised by providing the highest commercially available spatial resolution optical satellite data. MAXAR now provide up to 30cm spatial resolution imagery with their WorldView-3 and Worldview-4 satellites.
But spatial resolution can refer to a number of related factors, which are important to be aware of when working with VHR imagery.
It can refer to the size of the objects visible in the image, the pixel size of the raster image and/or the collected ground sample distance.
Pixel size
VHR data is stored in raster image format, which is a regular grid of pixels.
The linear unit of measure that a raster pixel represents on the ground in the real world, is it's pixel size, or resolution.
Pixel size can dictate the minimum size of homogenous objects or features which can be seen in the image.
In general, the object or feature would have to be the same size or larger than the claimed resolution. However, in some cases smaller features may also be possible to detect if their reflectance dominates against the background.
Ground Sample Distance (GSD)
An important parameter to consider when using the imagery, is the collected Ground Sample Distance (GSD). This is the measure of the distance between each pixel centre on the ground at data acquisition.
The GSD is not consistent for a given satellite platform, nor is it consistent across a single image.
GSD will vary dependent upon the angle at which the satellite was pointing towards the ground at the time of data acquisition, and the steepness of the terrain it is imaging.
A VHR satellite platform will rarely, if ever point directly downwards to the ground. It will point at an angle towards it's target area. This is known as the angle off-nadir. 0 degrees would denote the satellite was pointing directly downwards. A higher off-nadir angle indicates the satellite was pointing at a greater angle away from directly downwards. If the terrain was completely flat, this would translate to a larger footprint of area captured on the ground per pixel, meaning a larger GSD.
In practice this means that the true spatial resolution of the highest resolution imagery is usually between 33 and 40 cm. The average collected GSD is available in the metadata provided with MAXAR products.
MAXAR have recently developed a new high definition imagery product at 15cm spatial resolution. It is important to be aware that the data that underpin this product has not been collected at 15cm ground sample distance (GSD). Instead MAXAR have applied a proprietary algorithm to their 30cm GSD collected imagery to 'sharpen' it. You can read more information about it on the MAXAR blog. Whilst this is great for it's potential to improve how well we can discern features on the ground, it may have particular considerations when using the data for analytical applications. More information and explanations are available here.
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