Home > Public Domain Data > Be Critical of the Data: Imagery too!

Be Critical of the Data: Imagery too!

As we have written about many times in this blog and in our book, being a critical consumer of data is essential for successful, wise decision making in the modern world.  Geospatial data offers a vast array of capabilities, but also offers numerous examples of discrepancies that often are found only through paying attention to details.  Even imagery is not immune to offsets and discrepancies, as we have detailed here and here.  The following represents another example of why this all matters.

A colleague of mine was working on a project outside of the town of Redcliff, in western Colorado USA.   In the first image, the red point on the older World Imagery (Esri Clarity) layer is at the corner of the house, and its coordinates (in WGS84) were calculated on that image.  In the second image, the white point at the corner of the house is on the current Esri World Imagery layer, and the coordinates (in WGS84) that were calculated based on that image.

bonar1
bonar2

Below are the same points on Google Earth. They match up pretty well with the World Imagery (Clarity) imagery.

bonar_google_earth

Further investigating the extent of what seems to be an offset between the two image layers are three screen captures below in the Paonia, Colorado area. These show a Delta County local road crossing the North Fork Gunnison River.   The map is cast in NAD83 UTM Zone 13.  The roads shapefile is from the Colorado Department of Transportation, Delta County Local and Major Roads (2019), and also in NAD83 UTM13.  The intersection point shows WGS1984 Lat-Long coordinates.   The two Esri World Imagery layers are both in WGS_1984_Web_Mercator_Auxiliary_Sphere.   The latest USDA NAIP (2019) is in NAD83 UTM13.   The World Imagery (Clarity) basemap and the NAIP both line up well with the Colorado DOT roads shapefile. The Esri World Imagery basemap does not.

bonar5
bonar6
bonar7

After reading these notes from my colleague, I spent some time looking at this, and then, along the lines of “being critical of the data” I did some of my own investigating.  I drew the yellow line in ArcGIS Online on the image along the bridge following the Clarity layer, and a cream-colored line along the bridge following the base imagery layer, noting that about a 12 meter offset exists, largely along a north-south axis, here.  But, I panned to other locations in Colorado and outside Colorado and found no offset.

bonar3
bonar4

I relayed this to my Esri colleagues who upon investigation, told me that this is not a projection issue, but likely a shift introduced in Maxar’s orthorectification process (related to ground control or DEM or both).   They told me that they will be publishing CONUS wide updates over the next several months which should resolve this particular localized shift.   Hence, repeat my above experiment whenever you happen to be reading this essay, and you may find different (and hopefully better) results!

I think at least five take-away points are important here (feel free to add more in the comments section).

(1)  GIS and remote sensing tools and data are continually evolving particularly in our data-as-services world.

(2)  Sometimes a wealth of information can be obtained by asking questions of the data and services providers.  Sometimes they can even hasten changes that need to be made.

(3)  The user needs to determine “fitness for use” regarding the spatial accuracy, completeness, date, and other characteristics for their project:  The project’s needs determine whether the data under consideration will be useful.  In the above example, if you were laying water or gas pipe or conducting land surveying for the assessor’s department, the above offsets would likely require you to use the Clarity layer or seek another source.  But if you are assessing regional or even local land use patterns, either layer would likely be just fine.

(4) Be critical of the data.   Be curious and ask questions.

(5)  Pay attention to detail.  Investigate.

–Joseph Kerski

Categories: Public Domain Data Tags: ,
  1. Rachel Summers
    August 12, 2020 at 2:02 am

    This is an excellent investigation into local artifacts that may affect your working with data. I promote this kind of investigation to my students whenever I can. I always say ‘know your data’.

    • josephkerski
      August 12, 2020 at 1:57 pm

      Thanks Rachel and hope you are doing well! Yes, know your data!! –Joseph Kerski

  2. josephkerski
    February 10, 2021 at 9:15 pm

    Thank you for sharing this article!

  1. February 10, 2021 at 9:14 pm
  2. May 24, 2021 at 3:46 pm

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: