It is not uncommon that users of satellite imagery require the exact collection date and time of their data. And while all satellite imagery is originally delivered with metadata that contains this information, it is admittedly not the easiest to find. As such, in this two-part Geospatial Tip of the Month (GTM) I will show the process of finding the collection date and time for the most common satellite systems, including the DigitalGlobe satellites and RapidEye in part one; and then the Airbus satellites, EROS B and KOMPSAT-3 in part two.
One huge advantage of satellite imagery is that it is collected on a known date at a precise time, and this information is bundled with the data that is beamed back to the planet. Therefore, with all satellite imagery files, the operators deliver metadata files that contain this timing information and much more. And while I cannot assure you that your satellite imagery will still be contained in a folder with these metadata files, if they are, the steps below will get you working with these files so you can find the date and time stamp your project requires. So without any more delay, let’s up into the first part of this GTM.
DigitalGlobe Satellites’ Metadata
Metadata files for DigitalGlobe satellites are originally contained in the same folder as the large imagery TIFF file(s) you downloaded. The one you are looking for ends in the extension .IMD. The file name is nearly identical to that of the TIFF file only the R1C1, R2C1, etc. will not be present. So for example, the IMD file might have this full name:
Once you find this file, you can open with any text editor, I prefer to use the light-weight Microsoft Windows application, WordPad. WordPad is a nice choice as most/all PC users will have it and it maintains text formatting better than does Notepad so it is easier to find the info you need. Scroll down through the IMD file and towards the bottom you will find these two lines (these are examples from a file I am looking at now so your date and times will be very different):
earliestAcqTime = 2014-09-24T10:50:08.231005Z;
latestAcqTime = 2014-09-24T10:50:08.231005Z;
The first line tells you the date (e.g. 2014-09-24, Year-Month-Day) and exact time when the satellite started scanning your imagery. The second line tells you when it stopped imaging. You will need to convert the time (e.g. 10:50:08.231005Z, Hours:Minutes:Seconds) to your local time zone as these values are in UTC (some call it GMT). 24-hour or military time is used as well. This metadata format applies to all DigitalGlobe satellites including GeoEye-1, QuickBird and WorldView-1, 2 and 3.
For 80-centimeter panchromatic and 3.2-meter multispectral IKONOS data, the collection date and time could be found in a TXT file with a name like this:
The first part of the name (the order number in case you were curious) would be the same as the first part of the TIFF file name, and was originally located in the same folder as the TIFF. You can open the TXT file with any text editor such as WordPad. You will scroll down, about half way this time, to locate this example line:
Acquisition Date/Time: 2002-06-09 13:27 GMT
The time stamp for IKONOS data is not as exact as other DigitalGlobe satellites but is accurate down to the minute (e.g. 13:27, Hours:Minutes). Please note that if you have IKONOS and/or GeoEye-1 data ordered when these satellite were owned by GeoEye, the metadata will look different but will still be found in a text file (possibly an XML, sorry it has been too long to remember which it is) in a very similar location and naming convention.
Similar to the satellites explored above, 5-meter multispectral RapidEye metadata files were originally located in the same folder that contained the TIFFs when downloaded from FTP. A slight difference would be the file format as RapidEye metadata is in XML format. I typically use Internet Explorer to open XML files. You will be looking for a file named like this:
Your file name will have a different starting string of numbers and letters (it will match the file name of the TIFF) with _metadata appended to it. Once you open the XML file, you will look for the following lines close to the top:
Similar to the DigitalGlobe satellites, the collection date (e.g. 2011-06-28, Year-Month-Day) and time (e.g. T18:57:36Z, Hours:Minutes:Seconds) for RapidEye imagery is in UTC so you will need to convert to your local time zone. You are given both the start and stop times for the acquisition of the image down to second.
If you have time, check out the short video that accompanies this GTM for a visual description of how to find the collection date and time for DigitalGlobe and RapidEye satellite imagery. In our next edition of the GTM, I will finish this topic by showing you how to find the same date/time information for Airbus satellites, EROS B and KOMPSAT-3.
Do you have an idea for a future GTM? If so, let me know by email at firstname.lastname@example.org.
Brock Adam McCarty