Outstanding Science!

It’s British Science Week! Co-ordinated by the British Science Association (BSA) and funded by the UK Government through the Department for Business, Energy and Industrial Strategy, it’s a celebration of science, engineering, technology and maths – often referred to as STEM.

The week runs from 10th to the 19th March which technically makes it a ten day festival – a slightly concerning lack of precision for a celebration of these subjects! There are events taking place all over the UK, and you can see here if there are any local to you. For us, there are nine events taking place in Plymouth. Highlights include:

  • Be a Marine Biologist for A Day running on the 16th and 17th at the Marine Biological Association
  • Science Week Challenge – Cliffhanger: On 17th of March teams of students from Secondary Schools across Plymouth will compete to design and build a machine to solve a problem.
  • Dartmoor Zoological Park running a STEM careers day. Although, sadly you’ve already missed this as it took place on Tuesday!

All of these, and the many others across the country, are fantastic for promoting, educating and inspiring everyone to get involved with STEM subjects and careers. Regularly readers know this is something that we’re very keen on at Pixalytics. Eighteen months ago we published a book, ‘Practical Handbook of Remote Sensing’, which aims to take complete beginners through the process of finding, downloading, processing and visualising remote sensing satellite data using just their home PC and an internet connection.

We were delighted to find out recently that our book has been chosen an Outstanding Academic Title (OAT) of 2016 by Choice, a publication of the Association of College & Research Libraries, a division of the American Libraries Association.

OAT’s are chosen from titles reviewed in Choice over the last year, and selected books demonstrate excellence in scholarship, presentation and a significant contribution to the field. The reviewer’s comments are integral to this process. Someone from San Diego State University reviewed our book last August and their comments included:

  • ‘a unique approach to the presentation of the subject’
  • ‘This book is successful in achieving its aim of making the science of remote sensing accessible to a broad readership.’
  • ‘Highly recommended. All library collections’

OAT’s are a celebration of the best academic books and Choice selected 500 titles out of 5,500 they reviewed last year. We’re very proud to have been included in this list.

Everyone can, and should, get involved in science. So why not go to one of the British Science Week events local to you, or if not you could always read a book!

Plymouth Student’s Shot at Space!

From left to right: Fraser Searle (President), Sam Kennerley (Secretary) of Plymouth University Space Society, with the equipment to launch the balloon.

Plymouth University’s Space Society is planning to send a small shot glass ‘into space’ attached to a weather balloon in the coming week.

The aim is to send the glass 100,000 feet above the Earth, equivalent to 30 kilometres, and then bring it back safely. On its return, in true student fashion, they intend to use the glass to drink a few ‘space cocktails’!

The idea for launching the weather balloon began last summer when Fraser Searle and Nick Hardacre, who lead the Space Society at Plymouth University, were looking for ways to create interest in space in the local community. They originally hoped to send a bottle of local gin up, but soon found the challenges of working in a sub zero environment. It would have taken a balloon one and half times the size of the current one and double the volume of helium, so they changed to the shot glass.

They’ll also be attaching cameras and tracking equipment to the six metre diameter balloon to record and monitor the journey. The students have a roller coaster of emotions at the moment as Fraser explained, “We’re feeling excited, but I do get waves of nerves as to whether the glass and the cameras will return unharmed. We’re also wondering if the pictures and videos will be clear.”

Technically, the weather balloon won’t get into space. It should reach the upper half of the stratosphere, an area known as near space. As this area stretches from 20km to 100km above the Earth, ‘near’ is a relative term.

Pixalytics got involved with the project before Christmas, when we helped with sponsorship to enable the students to finish purchasing the necessary equipment. We’re also hoping to provide support in reviewing and interpreting the images the cameras collect on the journey. It’ll be interesting to compare what the weather balloon sees, with what various satellite imagery shows.

We’re strong supporters of events that encourage students and early career scientists to enhance their understanding of remote sensing, space and science. We sponsor student conferences and prizes that take place in the UK. So, it’s fantastic to get involved in something much closer to home.

Launching a weather balloon requires permission from the Civil Aviation Authority, and is also highly weather dependent. A planned launched at the end of January had to be abandoned as the balloon was likely to end up in Portsmouth or Calais harbour.

However, the team have once again got the relevant permissions to try again this coming week. The exact launch date will depend on the wind and weather patterns around Plymouth, which are always fairly turbulent. Fraser said, “We’ll be glued to the online predictors to find a launch slot.”

This is great local project for Plymouth, and we’re pleased to be able to support it. We have our fingers crossed for suitable weather, but only time will tell if they manage to conquer space!

Spinning Python in Green Spaces

2016 map of green spaces in Plymouth, using Sentinel-2 data courtesy of Copernicus/ESA.

2016 map of green spaces in Plymouth, using Sentinel-2 data courtesy of Copernicus/ESA.

As students, we are forever encouraged to find work experience to develop our real-life skills and enhance our CV’s. During the early period of my second year I was thinking about possible work experience for the following summer. Thanks to my University department, I was able to find the Space Placements in INdustry (SPIN) scheme. SPIN has been running for 4 years now, advertising short summer placements at host companies. These provide a basis for which students with degrees involving maths/physics/computer science can get an insight into the thriving space sector. I chose to apply to Pixalytics, and three months later they accepted my application in late March.

Fast forward a few more months and I was on the familiar train down to Plymouth in my home county of Devon. Regardless of your origin, living in a new place never fails to confuse, but with perseverance, I managed to settle in quickly. In the same way I could associate my own knowledge from my degree (such as atmospheric physics, and statistics) to the subject of remote sensing, a topic which I had not previously learnt about. Within a few days I was at work on my own projects learning more on the way.

My first task was an informal investigation into Open data that Plymouth City Council (PCC) has recently uploaded onto the web. PCC are looking for ways to create and support innovative business ideas that could potentially use open data. Given their background, Pixalytics could see the potential in developing this. I used the PCC’s green space, nature reserve and neighbourhood open data sets and found a way to calculate areas of green space in Plymouth using Landsat/Sentinel 2 satellite data to provide a comparison.

Sentinel-2 Image of Plymouth from 2016. Data courtesy of Copernicus/ESA.

Sentinel-2 Image of Plymouth from 2016. Data courtesy of Copernicus/ESA.

There were a few challenges to overcome in using the multiple PCC data sets as they had different coordinate reference systems, which needed to be consistent to be used in GIS software. For example, the Nature Reserves data set was partly in WGS84 and partly in OSGB 1936. Green space is in WGS 84 and the neighbourhood boundaries are in OSGB 1936. This meant that after importing these data sets in GIS software, they wouldn’t line up. Also, the green space data set didn’t include landmarks such as the disused Plymouth City airport, and large areas around Derriford Hospital and Ernsettle. Using GIS software I then went on to find a way to classify and calculate areas of green space within the Plymouth city boundary. The Sentinel-2 which can be seen above, has a higher spatial resolution and allowed me to include front and back gardens.

My green space map for 2016 created from Sentinel 2 data is the most accurate, and gives a total area of green space within the Plymouth neighbourhood boundary of 43 square kilometres, compared with 28 square kilometres that PCC have designated within their dataset. There are some obvious explainable differences, but it would be interesting to explore this deeper.

My second project was to write computer code for the processing and mosaicking of Landsat Imagery. Pixalytics is developing products where the user can select an area of interest from a global map, and these can cause difficult if the area crosses multiple images. My work was to make these images as continuous as possible, accounting for the differences in radiances.

I ended up developing a Python package, some of whose functions include obtaining the WRS path and row from an inputted Latitude and Longitude, correcting for the difference in radiances, and clipping and merging multiple images. There is also code that helps reduce the visual impact of clouds on individual images by using the quality band of the Landsat 8 product. This project took up most of my time, however I don’t think readers would appreciate, yet alone read a 500 line python script, so this has been left out.

I’d like to take this opportunity to thank Andrew and Samantha for giving me an insight into this niche, and potentially lucrative area of science as it has given me some direction and motivation for the last year of my degree. I hope I’ve provided some useful input to Pixalytics (even if it is just giving Samantha a very long winded Python lesson), because they certainly have done with me!

 

Blog written by:
Miles Lemmer, SPIN Summer Placement student.
BSc. Environmental Physics, University of Reading.

Merry Christmas!

Plymouth by night taken from the International Space Station, Feb 13

Plymouth by night taken from the International Space Station, Feb 13
Image courtesy of http://www.citiesatnight.org/

Pixalytics Ltd would like to wish everyone we worked with during the last year, and all the readers of our blog, a very happy Christmas.

Back next week with our top blogs of 2015!

The Road To Success….

Danube river crossing The Great Romanian Flood Plain. Image acquired by Sentinel-2A on the 3rd December 2015. Data courtesy of ESA.

Danube river crossing The Great Romanian Flood Plain. Image acquired by Sentinel-2A on the 3rd December 2015. Data courtesy of ESA.

‘On the road, you will face many stumbling blocks, twists, and turns… You may never know how far the road will take you.’ **

In my case, the road brought me to Plymouth, a city on the south coast of Devon, England, a magical place with great history and outstanding views.

What I am doing here? Well, I am pursuing my dream of becoming a GIS and Remote Sensing Specialist by doing an internship through the Erasmus + programme at a local company called Pixalytics. My mentor is Dr. Samantha Lavender, is a great professional with vast experience in this field, She is also the Chairman of the British Association of Remote Sensing Companies and former Chairman of the Remote Sensing & Photogrammetry Society. For me, this is about more than just getting a grade, earning credit, or making money; this is an opportunity to learn, ask questions, and impress with my eagerness.

Finding this internship was easy for me. With a short search on Google I found this Pixalytics blog, where a previous student here had posted her impressions and thoughts on the company. I immediately said “This worth trying!” In the next moment I opened my email started writing, I sent wrote emails to multiple addresses, to make sure my message reached the target. After just two days, I received an answer from Mr. Andrew Lavender and it was positive!

I was very happy and because I knew the departure papers would take over a month to be completed, I immediately started doing them. All of this happened at the end of September. After my papers were done, I bought my flight ticket to Luton Airport, then a bus to London and then onto Plymouth. I arrived on December 5th and so, like the previous student, here I am posting my own impressions and thoughts on the Pixalytics blog page.

My first day at Pixalytics started pretty badly, I got lost and arrived a little late. I now remind myself each morning to turn left, not right, when I get off the bus. I got a short introduction to the building where the company is located, and my office for the next three months, which by the way looks very good. The office has a professional, but relaxed, atmosphere and I soon started working, one of my first tasks being the downloading of Sentinel-2A data, which proved a very difficult one due to slow data speeds and functionality of the ESA Data Hub.

Over the next three months, I am expecting to assist Pixalytics in developing their agritech products, explore the potential of Sentinel-2A data and I will be doing my own research into Urban Sprawl in Romania. I am hoping to have the opportunity to present my research at a conference during my placement.

It has been over a week now since I came to Plymouth and I feel great, working at Pixalytics is a great opportunity for my career and I will take full advantage of this. I strongly recommend all students who want to burst their work experience and who want to see what it is like to be in a professional business environment, to search for Erasmus+ placement offers as I did. You will not regret it!

Blog written by Catalin Cimpianu

** Quote is by Tony Hassini, from ‘The Road To Success’

Sentinel-2A Data Released Into The Wild

False Colour Image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

False Colour Image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

Sentinel -2A is already producing some fantastic images, and last week ESA announced the availability of Sentinel-2A orthorectified products in the Sentinel Data Hub. This will enable Sentinel-2 data to be accessed more widely, although as we found out this week there are still a few teething problems to sort out.

At the top of the blog is a stunning image of the Chinese city of Qingdao, in the eastern Shangdong province. The false colour image shows the city of Qingdao and the surrounding area with the centre dominated by Jiaozhou Bay, which is natural inlet to the Yellow Sea. The bay is 32 km long and 27 km wide, and generally has a depth of around ten to fifteen metres; although there are deeper dredged channels to allow larger ships to enter the local ports. The bay itself has decreased by around 35% since 1928, due to urban and industrial growth in the area.

Jiaozhou Bay Bridge a sub-set of a false colour image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

Jiaozhou Bay Bridge a sub-set of a false colour image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

There is a tenuous linguistic link between Plymouth, where Pixalytics is based, and Qingdao. Plymouth is branded as Britain’s Ocean City and Qingdao is home to the Ocean University of China. Qingdao does however, have a much greater claim to fame. It is home to the World’s Longest Bridge. The Jiaozhou Bay Bridge is 42 km long and transects the bay. It is clearly visible on the satellite image, although you might not be able to see it on the thumbnail image at the top of the blog. Therefore, if you look at the subset to the right, you should be able to see bridge clearly and boats on the bay.

Now Sentinel-2A data has been released into the Sentinel Data Hub, images like this are waiting for everyone in the world to discover. We’ve been testing Sentinel-2A data for a few months already, as were part of the community who gave feedback to ESA on the quality of the data. Sentinel-2A carries a Multispectral Imager (MSI) that has 13 spectral bands with 4 visible and near infra-red spectral bands with a spatial resolution of 10 m, 6 short wave infrared spectral bands with a spatial resolution of 20 m and 3 atmospheric correction bands with a spatial resolution of 60 m. When the identical Sentinel-2B is launched in late 2016, the pair will offer a revisit time of only 5 days.

The data from Sentinel-2A forms part of the Copernicus program and is freely available to use, as such it is bound to be very popular. So popular in fact, we found it difficult to get on the Data Hub this week, with slow data speeds and a few elements of the functionality not working efficiently. Although, we’re sure that these will be resolved quickly. Also, there are user guides and tutorials available on the website to help people use the data hub.

The Sentinel-2A data release, following on from the microwave data from Sentinel-1, is a watershed moment for Earth Observation companies, given their spatial resolution, revisit time and free availability, they offer a unique opportunity to develop satellite data services. We’re intending to use this data, are you?

Footprints in Remote Sensing

Plymouth Sound on 25th July 2014 from Landsat 8: Image courtesy of USGS/NASA Landsat

Plymouth Sound on 25th July 2014 from Landsat 8: Image courtesy of USGS/NASA Landsat

I’ve just finished my summer with Pixalytics! As I wrote a blog when I first arrived, I thought it would be nice symmetry to finish my ERASMUS+ placement with a second one.

When I started my internship, I had very little real-world experience. I was really excited and really nervous, but this internship has been a huge eye opener for me. I spent the first week understanding and reviewing the practicals within Pixalytics’ forthcoming book ‘The Practical Handbook of Remote Sensing’ to check for any errors prior to publication, which gave me a good understanding of the basics of remote sensing.

Over the next few weeks I applied my new knowledge to finding and downloading Landsat data for a commercial client. I then downloaded additional Landsat datasats and compared them to altimetry datasets to look for patterns between the two sources for the NovaSAR project. My other main job was processing Landsat 8 data to create a UK-wide vegetation mosaic. This needed cloud free images which is really difficult because the weather in UK is always cloudy, even in summer!

Plymouth is a deeply captivating city with astonishingly magnificent views and landscapes. You get the urban city, fantastic scenery and all around Plymouth are nice beaches, cities and the Dartmoor National Park which is always worth a visit. It’s a safe quiet place where everything is so close together that you can walk everywhere. The people are generally friendly and warm-hearted, and the experience of living in the Plymouth for two months has helped me to gain a more fluent level of English and a better understanding of the British culture – I now know why they constantly talk about the weather!

Overall, I’ve learnt a lot from the internship including practical skills that I will be able to carry with me to my next position. Needless to say, I will miss Pixalytics and Plymouth very dearly, and I’m thankful for the chance to work and live there. ERASMUS+ is an great opportunity that everyone should try to be part of, and I totally recommend going abroad because is an experience that stays with you to rest of your life.

Bye Plymouth, Bye Pixalytics!

Selin

Blog by Selin Cakaloglu, Erasmus+ Intern at Pixalytics

Evolution of Coastal Zones

Lost Lake Area of Louisiana, USA. Landsat 5 image from 1985 on left, Landsat 8 from 2015 on right. Data courtesy of NASA/USGS.

Lost Lake Area of Louisiana, USA. Landsat 5 image from 1985 on left, Landsat 8 from 2015 on right. Data courtesy of NASA/USGS.

Coastal zones are the place where the sea and the land meet, and they’ve played a massive role in the life of Pixalytics. From a personal standpoint we’re based, and live, in Plymouth on the south-west coast and anyone who saw the Dawlish railway tracks swinging in midair eighteen months ago will know how these areas can affect our transport links. In addition, Sam’s PhD was focussed on the ‘Remote Sensing of Suspend Sediment in the Humber Estuary’, and so Pixalytics has effectively been grown from a coastal zone!

Last week the BBC carried a report highlighting the erosion of the Louisiana coastal wetlands; in particular, it noted that more than an area the size of a football pitch was disappearing every hour. This statistic caught our attention, and our next steps were obvious! We downloaded two images of the Lafourche Bayou in Louisiana; the first was a Landsat 5 image acquired on the 31st August 1985, and the second was a Landsat 8 image acquired twenty years later on the 02nd August 2015.

Mouth of Atchafalya River, Louisiana, USA. Landsat 5 image on left from 1985, Landsat 8 image from 2015 on right. Data courtesy of NASA/USGS.

Mouth of Atchafalya River, Louisiana, USA. Landsat 5 image on left from 1985, Landsat 8 image from 2015 on right. Data courtesy of NASA/USGS.

The image at the top of the blog shows the area around the Lost Lake, in the bottom left hand corner, just off the coast of Louisiana; with the 1985 image on the left, and the 2015 image on the right. The loss of land, described in the BBC report, can be seen in the northern portion of the image with a lot more water visible. However, the image on the right shows the mouth of the Atchafalya River in Louisiana; again, the 1985 image is on the left. Coastal evolution is again clearly visible, but this time there are islands that have risen from the water.

Swamplands, like in Louisiana, aren’t the only coastal zones changing. In 2011, the United Nations Environmental Programme estimated that over the last 40 years Jamaica’s Negril beaches have experienced average beach erosion of between 0.5 m and 1 m per year. Another coastal zone in decline are mangroves and wetland forests; a 2007 report noted that the areal extent of mangrove forests had declined by between 35 % and 86 % over the last quarter half century (Duke et al. 2007).

Coastal zones have social, economic and environmental importance as they attract both human settlements and economic activity; however, they are also particularly susceptible to the impacts of climate change and their evolution will have impacts on the human, flora and fauna populations of those areas. So when you’re next at the coast have a good look around; the view in front of you may never be seen again!

First Small Steps in Remote Sensing

The International Space Station is seen in silhouette as it transits the moon at roughly five miles per second, Sunday, Aug. 2, 2015, Woodford, VA.  Photo Credit: (NASA/Bill Ingalls)

The International Space Station is seen in silhouette as it transits the moon at roughly five miles per second, Sunday, Aug. 2, 2015, Woodford, VA. Photo Credit: (NASA/Bill Ingalls)

It’s not often you get given the opportunity to travel, live in an exciting new city and get an incredible internship all in one. So when I heard about the Erasmus+ Programme I applied right away! I wanted to gain more experience in remote sensing.

When I was little I had a very big poster of the moon surface hung on my wall, it had so much detail and I would stare at it every night before I went to bed. After my parents bought my first computer, I started to search for more images of the moon and other planets and I was impressed by the complexity of what I found. This was the beginning of my fascination with remote sensing. When it came to choosing my career path, it was not hard. I knew what I wanted to become and now it sounds, and feels, right to call myself a Geomatics Engineer.

I’m currently studying two undergraduate degrees in Surveying, and Civil Engineering; but it was still hard to find an Erasmus work placement for remote sensing. I managed to find the Pixalytics Ltd with my teacher’s help, as he had previously met Dr Samantha Lavender.

After finding a place to do your internship the rest is should be easy, but not for United Kingdom. Getting my work permit from British Council was a really challenging process, and took me exactly three months. Despite doing everything right, getting responses to my emails for sponsorship was hard. It was the most awful part of the process for me, because there was nothing I could do except wait. Finally, after a lot of patience my visa arrived and I was on my way to Plymouth!

The last issue, and some people’s main concern, is getting accommodation. I did not find it hard to find a place to stay because most of the students were out of town. With a basic search on the internet I found a flat in four days, it is based a few hundred metres from the centre of Plymouth and close to the bus route to Pixalytics.

I thought I had read and traveled enough to be prepared when I stepped off the plane in London, but it was still a shock standing alone with my suitcase and hearing all the British accents around me. At first, it was difficult to adapt to the language as the accents are sometimes hard to understand. But once I’d grasped the pronunciation, I believe I’m improving every week.

Working at Pixalytics will be my first internship experience, and I am so grateful to Samantha Lavender for giving me this opportunity. Working abroad will be a memory and lesson in itself but I hope to also I hope to enhance my discipline and knowledge as well as applying my existing engineering and personal skills.

Getting my internship was a long, difficult and exhausting process, but I realized that it’s totally worth it as soon as I got to Plymouth, If anyone is thinking of applying to the Erasmus+ programme, I would totally recommend it!

Blog by Selin Cakaloglu, Erasmus+ Intern at Pixalytics

Five Landsat Quirks You Should Know

South West England from the 8th December 2014. Landsat 7 imagery courtesy of NASA Goddard Space Flight Center and U.S. Geological Survey

South West England from the 8th December 2014. Landsat 7 imagery courtesy of NASA Goddard Space Flight Center and U.S. Geological Survey

If you’ve started using Landsat after our five simple steps blog last week, or perhaps you’ve used its imagery for awhile, you may have come across, what we’ll call, quirks of Landsat. These may be things you didn’t understand, things that confused you or where you thought you’d done something wrong. This week we’re going to try to demystify some of the common quirks and questions with using Landsat data and imagery.

Quirk One: What do the WRS path and row numbers mean?
The Worldwide Reference System (WRS) is what Landsat uses to map its orbits around the world, and is defined by sequential path and row numbers. Despite its name, there are in fact two versions of the WRS; WRS-1 that’s used for Landsat’s 1-3, and WRS-2 for the rest of the missions.

The paths are a series of vertical-ish tracks going from east to west, where Path 001 crosses the equator at 65.48 degrees west Longitude. In WRS-1, there are 251 tracks, whereas the instruments in Landsat 4 and beyond have a wider swath width and only require 233 tracks to cover the globe. Both WRS-1 and WRS-2 use the same 119 Rows, where Row 001 starts near the North Pole at Latitude 80 degrees, 1 minute and 12 seconds north , Row 60 coincides with the Equator at Latitude 0, and row 119 mirrors the start at Latitude 80 degrees, 1 minute and 12 seconds south. A combination of path and row numbers gives a unique reference within Landsat, the path number always comes first, followed by the row number. For example, 204-025 is the WRS-2 path and row for Plymouth.

There are maps available of the paths and rows. However, there is also handy website from USGS that converts path and row numbers to Latitude and Longitude and vice versa; it’s accompanied by a map so you can tell you’ve got the area you want!

Quirk Two: My image has a minus one percent cloud cover
This one can be confusing! On the GloVis image selector you have the option to specify the maximum percentage of cloud cover on your image. Selecting 50% means up to 50% of the image could be cloud, and selecting 0% means no cloud at all.

Cloud cover is calculated using both the optical and thermal bands, and therefore as any Landsat imagery taken using the Multispectral Scanner System (MSS) does not include a thermal band, the cloud cover percentage is not easily calculated. Where a calculation does not occur the cloud cover percentage is set to -1%.

At the bottom of the Scene Information Box, there is line for Sensor/Product. Although, the title changes it effectively displays similar information. If the sensor/product line includes TM, ETM+ or OLI-TIRS, meaning Thematic Mapper, Enhanced Thematic Mapper Plus or Operational Land Imager-Thermal InfraRed Sensor respectively, the cloud cover will usually be calculated as all these sensors have a thermal band. Whereas, if the sensor/product is MSS, then the cloud cover percentage will be -1%.

Landsat 8 uses the OLI-TIRS sensor, Landsat 7 has the ETM+ sensor, whereas Landsat’s 4 & 5 have both TM and MSS sensors, and Landsat’s 1, 2 & 3 only have MSS.

Quirk Three: What are all the other files alongside the LandsatLook Natural Colour Image?
When you select an image from Landsat, you’re given all available Landsat products associated with it. The most common additional products you’ll be offered are:

  • LandsatLook Thermal Image – This is usually a jpeg of the thermal band, which shows the variations in temperature, where the darker areas are colder, and the lighter areas are warmer.
  • LandsatLook Quality Image – Currently only available with Landsat 8, and is a jpeg which shows the positions of the clouds and other features such as snow and ice on your image.
  • LandsatLook Images with Geographic Reference – These are a series of compressed data files which can be uploaded into a Geographical Information System, allowing the application of image processing techniques. These are big files compressed, an even bigger uncompressed, and so you need a lot of storage space if you start downloading them!

Quirk Four: Why do some Landsat 7 images have black stripes on them?

South West England from the 8th December 2014, showing black stripes.  Landsat 7 imagery courtesy of USGS/NASA.

South West England from the 8th December 2014, showing black stripes.
Landsat 7 imagery courtesy of USGS/NASA.

This is due to the failure of Landsat 7’s Scan Line Corrector on the 31st May 2003. The Scan Line Corrector’s role is to compensate for the forward movement of the satellite as it orbits, and the failure means instead of mapping in straight lines, a zigzag ground track is followed. This causes parts of the edge of the image not to be mapped; hence giving you the black stripe effect – it can be seen clearly to the right with a zoomed in version of the image at the top of the blog. The location of the black stripes varies, and each stripe represents between 390 – 450m of the image; therefore US Geological Survey (USGS) estimates that affected images lose about 22% of their data.

The centre of the image can still be used, however it’s more complicated to use Landsat 7 data after May 2003. It’s worth noting that on the sensor/product line in the Scene Information Box, it uses the notation SLC-off to indicate that the image was taken after the Scan Line Corrector failed.

Quirk Five: My image has brightly coloured single pixels

Landsat 5 MSS image acquired on 16 January 1997 via ESA receiving station. Image courtesy of USGS/NASA/ESA.

Landsat 5 MSS image acquired on 16 January 1997 via ESA receiving station. Image courtesy of USGS/NASA/ESA.

Brightly coloured single pixels that don’t match the surrounding area, is phenomena known as Impulse Noise; which is also seen with dark or missing pixels. An example of an image with this phenomena is shown on the right. Technical issues during the downlink from the satellite or during the transcription from tape to digital media are the most frequent causes. However, small fires on the ground can also show up as bright pixels that cause the same effect, although these are less frequent. As Landsat has a 30m spatial resolution, these aren’t campfires or barbecues; but are high temperature features such as brush burning, wildfires or gas flares.

Images heavily affected by Impulse Noise aren’t released into the USGS archive. Also it’s only visible when zoomed it, and selecting another image from a different date will mostly likely cure the phenomena.

We hope this quintet of quirks has explained some of the queries and questions you might have about using Landsat data, and if you’ve not come across any of these yet this should give you a heads up for when you do come across them.