

Libra – The Landsat imagery browser you will love - smit1678
http://www.developmentseed.org/blog/2015/01/22/announcing-libra/

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ovis
This is very welcome, as I've often found the official NASA/USGS tools to be
difficult to use in practice.

One thing that I don't understand is why coverage appears to be poor or non-
existant at high latitudes (say, above 70 degrees). Shouldn't there be a lot
of images here because of the near-polar orbit (i.e. high latitudes should be
sampled frequently)?

Also, a graticule would be handy, as it can be tricky to find a specific place
with the simple basemap.

But thanks very much for this tool!

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shutupalready
I was expecting the Landsat images to be something like satellite view in
Google maps. I realize that close-up views in Google maps are collected by
airplanes, not satellites. I'm not expecting to see individual cars and houses
in the Landsat images, but the images I looked at were very disappointing.

The ones I downloaded were TIFF files, 8k x 8k (or sometimes 16k x 16k),
16-bit grayscale, covering large areas (~100km square), and if I zoomed-in the
image quickly became blurry.

With photos that I take with a digital camera that are only 2k x 2k, I can
zoom in quite a lot and see a lot of detail. The Landsat images seem to be
_much_ larger than they need to be for the level of detail they provide.

EDIT: To give an example, here's San Francisco that I zoomed-in on and
cropped:

[http://imgur.com/IoRFgCu](http://imgur.com/IoRFgCu)

You can see that zooming more will not make it better. Is that the best we get
from Landsat?

Also, why does the download contain a dozen files (named xxxB1.TIF, xxxB2.TIF,
xxxB3.TIF, etc.) that seem to cover the same area?

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bullfightonmars
Landsat images aren't taken with the primary intention of showing human level
detail. The imaging sensors are designed to image groundcover. In fact landsat
doesn't strictly image visible light, it's imaging bands are in visible,
infrared and near infrared depending on the satellite used. [1]

This can be used for all kinds of agricultural, environmental, and climate
change research.

Having satellite images that are higher in resolution wouldn't be particularly
helpful for these purposes and would instead be counter to their purpose
because it would take so much longer to image any given area. With large
images you can, within several days have images of the entire earth which is
much more useful for any temporally sensitive research. To say nothing for the
computing and storage requirements.

[http://landsat.usgs.gov/band_designations_landsat_satellites...](http://landsat.usgs.gov/band_designations_landsat_satellites.php)

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shutupalready
Thank you, but I still don't get this part:

> The Landsat images seem to be much larger than they need to be for the level
> of detail they provide.

For the level of detail I'm seeing, the same information could be represented
in a 2MB download (rather than 600-700MB that's currently downloaded).

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jofer
Each image is a different band. Landsat imagery is multispectral imagery. For
Landsat 8 data there are 11 bands and for Landsat 7 data there are 8 (if I
recall correctly?). Each band is typically distributed as an uncompressed,
16-bit tif.

JPEG and similar compression techniques are very effective at compression
image data without losing _visual_ information. However, it's very important
not to use lossy compression (such as JPEG) for scientific data such as this.
Our eyes may not mind, but classification algorithms do.

For various reasons, Landsat data is typically distributed and stored as a zip
of uncompressed tiffs. The actual bit depth is often a bit lower than 16 bit
(depends on the band), but it's easiest to distribute it as 16 bit to avoid
the need for unusual file formats.

At any rate, you can certainly compress these images quite a bit. However,
lossless compression algorithms are always at a disadvantage compared to lossy
compression (e.g. JPEG). There are plenty of more advanced lossless
compression algorithms (e.g. JPEG2000 or MrSid for this exact use case) than
putting uncompressed images in a zip file. Nonetheless, they only shave
~10-20% (ballpark: it varies a lot) of the file size off compared to zip (they
have other huge advantages when it comes to accessing the data, though).

However, the data needs to be easily used by a wide range of applications and
users. It's easy to unzip a file and then have "raw" uncompressed images. For
distribution, the drop-off in usability just isn't worth the savings in file
size. Also, there are a lot of legacy applications out there that wouldn't
react well to the USGS suddenly changing the way it distributes Landsat data.

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Sprint
Any idea why temporal coverage is so spotty? According to
[http://landsat.usgs.gov/landsat8.php](http://landsat.usgs.gov/landsat8.php)
Landsat 8 repeats every 16 days, but for many of the spots there are only
images that are several months old.

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geotyler
It could be that Libra throws out images with cloud cover percentages that are
too high. For some areas with frequent cloud cover, that might mean that there
are only a handful of useable Landsat images each year.

~~~
Sprint
Oh how I am not a fan of the "non-descript icon only" trend... I had no idea
there was a filter. Thank you!

