Posts Tagged ‘Accuracy’

Be Critical of the Data: Imagery too!

July 12, 2020 5 comments

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.


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


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.


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.


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: ,

Be a Wise Consumer of Fun Posts, too!

December 12, 2019 12 comments

Around this time of year, versions of the following story seem to make their way around the internet:

The passenger steamer SS Warrimoo was quietly knifing its way through the waters of the mid-Pacific on its way from Vancouver to Australia. The navigator had just finished working out a star fix & brought the master, Captain John Phillips, the result. The Warrimoo’s position was LAT 0º 31′ N and LON 179 30′  W.  The date was 31 December 1899.

“Know what this means?” First Mate Payton broke in, “We’re only a few miles from the intersection of the Equator and the International Date Line”.  Captain Phillips was prankish enough to take full advantage of the opportunity for achieving the navigational freak of a lifetime.  He called his navigators to the bridge to check & double check the ships position.  He changed course slightly so as to bear directly on his mark.  Then he adjusted the engine speed. The calm weather & clear night worked in his favor. 

At midnight the SS Warrimoo lay on the Equator at exactly the point where it crossed the International Date Line! The consequences of this bizarre position were many:  The forward part (bow) of the ship was in the Southern Hemisphere & the middle of summer. The rear (stern) was in the Northern Hemisphere & in the middle of winter.  The date in the aft part of the ship was 31 December 1899.  Forward it was 1 January 1900.  This ship was therefore not only in two different days, two different months, two different years, two different seasons, but in two different centuries – all at the same time.

I have successfully used many types of geographic puzzles with students and with the general public over the years, and I enjoy this story a great deal.  But in keeping with our reminders on this blog and in our book to “be critical of the data,” reflections on the incorrect or absent aspects to this story can be instructive as well as heighten interest. The SS Warrimoo was indeed an actual ship that was built by Swan & Hunter Ltd in Newcastle Upon Tyne, UK, in 1892, and was sunk after a collision with a French destroyer during World War I in 1918.  Whether it was sailing in the Pacific in 1899, I do not know.

The version of this story on CruisersForum states that it is “mostly true.”  What lends itself to scrutiny?  Let us investigate a few of the geographic aspects in the story.

First, the statement, “working out a star fix” leaves out the fact that chronometers were used to work out the longitude, rather than a sextant.  (And I highly recommend reading the book Longitude by Dava Sobel).  Second, the International Date Line (IDL) as we know it today was not in place back in 1899.  The nautical date line, not the same as the IDL, is a de jure construction determined by international agreement. It is the result of the 1917 Anglo-French Conference on Time-keeping at Sea, which recommended that all ships, both military and civilian, adopt hourly standard time zones on the high seas. The United States adopted its recommendation for U.S. military and merchant marine ships in 1920 (Wikipedia).

Third, the distance from LAT 0º 31′ N and LON 179 30′ W to LAT 0º 0′ N and LON 180′ W is about 42 nautical miles, and the ship could have traveled at a speed of no more than 20 knots (23 mph).  Therefore, conceivably, the ship could have reached the 0/180 point in a few hours, but whether it could have maneuvered in such a way to get the bow and stern in different hemispheres is unlikely, given the accuracy of measurement devices at the time.  Sextants have an error of as at least 2 kilometers in latitude, and chronographs about 30 kilometers in longitude. Or, they could already have reached the desired point earlier in the day and not have known it.  Even 120 years later, in my own work with GPS receivers at intersections of full degrees of latitude and longitude, it is difficult to get “exactly” on the desired point:  Look carefully at the GPS receiver in my video at 35 North Latitude 81 West Longitude as an example.  An interesting geographic fact is that, going straight East or West on the Equator along a straight line, it is possible to cross the dateline three times (see map below).

Our modern digital world is full of fragments that are interesting if not completely accurate, but as GIS professionals and educators, I think it is worth applying “be critical of the data” principles even to this type of information.  The story is still interesting as a hypothetical “what could have happened” and provides great teachable moments even if the actual event never occurred.


The International Date Line (CC BY-SA 3.0,