As the saying goes, to all good things must come an end. And sadly, we say goodbye this month to our regular series, the Geospatial Frequently Asked Question (G-FAQ); but fear not my two dedicated readers (ha!), we are replacing the G-FAQ with a new series called the Geospatial Tip of the Month (GTM). The GTM will have a similar approach in that it will address geospatial, GIS, imagery, remote sensing, etc. topics, however the format will be more condensed with a focus on videos where appropriate. I will also provide more ‘in-line’ links for our readers to delve deeper into side topics that arise as we explore the broad theme in each GTM. Oh and there is a small too slim chance that the GTM will not be monthly but it is my goal to keep them as regular as possible. So without further ado, let’s jump headlong into this month’s GTM.
Loading Metadata into ArcGIS with IMD Files
While there are multiple satellite imagery sources on the market, one of the most popular is DigitalGlobe. Operating a six satellite constellation including, GeoEye-1, IKONOS, QuickBird and WorldView 1/2/3, DigitalGlobe is a leading provider of high-resolution imagery. With their data products comes a standardized text file, ending in the extension .IMD, which contains metadata about each of the imagery scenes delivered to a client. If you open one of these IMD files, you will find a wealthy of information about the collection geometry and times, processing kernels and much more. For a complete list of the metadata included in DigitalGlobe’s IMD files, please refer to this detailed PDF.
In the video that accompanies this GTM, we will walk you through the steps of loading an IMD to check out the metadata that it loads in Esri’s ArcGIS. This metadata can be important for imagery users as they being to work with and analyze their DigitalGlobe datasets. From what we can tell, ArcGIS uses the metadata contained in these IMD files to improve mosaicking of multiple imagery datasets and very likely in pansharpening (though I cannot find evidence to back up this assumption anywhere). Admittedly, we use ArcGIS to work with vector datasets most commonly as it is not as powerful as Exelis’ ENVI or PCI Geomatics with regards to raster processing. So it is impossible for me to test how this metadata impacts mosaicking (and more) as we lack the licenses to do this. Here is a detailed handling of orthorecification and mosaicking of raster files using IMD metadata with ArcGIS.
A few additional points here on metadata before we move on to loading tiled imagery in ArcGIS:
- The information provided by Esri suggests that it is possible to load metadata with a wide variety of satellites, however we tested this on our end and had no luck. It is likely that you need to load this metadata info in the mosaicking process hence I could not find a way to do so.
- While IMD files are specific to DigitalGlobe sensors, keep in mind that IKONOS and GeoEye-1 were owned by GeoEye in the past. As such, if you have legacy files from these satellites, they will not contain IMD files to load metadata in ArcGIS.
- Every imaging company that owns a satellite provides metadata in their own custom format. Here is detailed information about the metadata contained with Pléiades 1 imagery, our other favorite high-resolution satellite.
- And finally, my use of the expression, imagery metadata, should not be confused with US government FGDC metadata standards. You can find out more about FGDC imagery metadata standards here.
Loading Tiled DigitalGlobe Imagery Products in ArcGIS
If you have ever ordered a large imagery dataset from DigitalGlobe, you likely found it divided into multiple tiles referred to as R1C1, R1C2, R2C1, etc. This is done for several reasons. First, the traditional GeoTIFF format has a file size limit of 4 gigabytes (GB) so tiling a large imagery order gets around this size limit. And second, loading one huge, high-resolution imagery file can take some time and it also can cause ArcGIS to run very slowly as you zoom and pan across the data; tiling helps to speed up these processes. That said, tiling can be annoying, especially if you have a huge number of tiles to load and work with in ArcGIS.
DigitalGlobe delivers a .TIL file with all imagery products that have been tiled out. This file is easy to load in ArcGIS as we will show you in the video here. Note that none of the other imagery companies currently provide such a file but I would not be surprised if that were to change in the future.
As a quick side note to this discussion, on the Esri website referenced above, it mentions that an IMD file should also load a tiled DigitalGlobe imagery product with the associated metadata. Unfortunately, I tested a wide variety of DigitalGlobe datasets and I found no way to load a tiled product with one button push as well as the metadata – perhaps it is a version issue on my end or a misstatement by Esri.
A short YouTube video showing you how to load DigitalGlobe imagery using a TIL and/or IMD file. I have also shown you a few reasons why loading data with the TIL/IMD files makes more sense than loading individual TIFF files. Happy Watching!
Do you have an idea for a future GTM? If so, let me know by email at email@example.com.
Brock Adam McCarty