Archive

Posts Tagged ‘web GIS’

Data Quality on Live Web Maps

June 19, 2017 3 comments

Modern web maps and the cloud-based GIS tools and services upon which they are built continue to improve in richness of content and in data quality.  But as we have focused on many times in this blog and in our book, maps are representations of reality.  They are extremely useful representations, to be sure, particularly so in the cloud, but still are representations.   These representations are dependent upon the data sources, accuracy standards, map projections, completeness, processing and rendering procedures used, regulations and policies in place, and much more.  A case in point are offsets between street data and the satellite image data that I noticed in mid-2017 in Chengdu in south-central China.  The streets are about 369 meters southeast of where they appear on the satellite image (below):

china-google-maps

Puzzled, I panned the map to other locations in China.  The offsets varied, but they appeared everywhere in the country; for example, note the offset of 557 meters where a highway crosses the river at Dongyang, again to the southeast:

china-google-maps2

As of this writing, the offset appears in the same cardinal direction and only in China; indeed; After examining border towns with North Korea, Vietnam, and other countries, the offset appears to stop along those borders.  No offsets exist in Hong Kong nor in Macao.  Yahoo Maps Bing Maps both show the same types of offsets in China (Bing maps example, below):

china_bing

MapQuest, which uses an OpenStreetMap base, showed no offset.  I then tested ArcGIS Online with a satellite image base and the OpenStreetMap base, and there was no offset there, either (below).  This offset is a datum issue related to national security that is documented in this Wikipedia article.  The same data restriction issues that we discuss in our book and in our blog touch on other aspects of geospatial data, such as fines for unauthorized surveys, lack of geotagging information on many cameras when the GPS chip detects a location within China, and seeming unlawfulness of crowdsourced mapping efforts such as OpenStreetMap.

But furthermore, as we have noted, the satellite images are processed tiled and data sets, and like other data sets, they need to be critically scrutinized as well.  They should not be considered “reality” despite their appearance of being the “actual” Earth’s surface.  They too contain error, may have been taken on different dates or seasons, may be reprojected on a different datum, and other data quality aspects need to be considered.

china-agol

Another difference between these maps is the wide variation in the amount of detail in terms of the streets data in China.  The OpenStreetMap was the most complete; the other web mapping platforms offered a varying level of detail; some of which were seriously lacking, surprisingly especially in the year 2017, in almost every type of street except major freeways.  The streets content was much more complete in other countries.

It all comes back to identifying your end goals in using any sort of GIS or mapping package.  Being critical of the data can and should be part of the decision making process that you use and the choice of tools and maps to use.  By the time you read this, the image offset problem could have been resolved.  Great!  But are there now new issues of concern? Data sources, methods, and quality vary considerably among different countries. Furthermore, the tools and data change frequently, along with the processing methods, and being critical of the data is not just something to practice one time, but rather, fundamental to everyday work with GIS.

Copyright in Today’s Web Mapping World

September 27, 2015 1 comment

GIS professional Nicholas Duggan has written an excellent article and a flowchart to help mapmakers and GIS analysts decide if they can legally use specific data sets in their work.

As Duggan points out, “anyone can make maps”, and as we emphasize in our book, today’s web mapping environment makes accessing data easier than ever.  But even though it is possible to use a specific data set, does that mean that we legally have the right to do so?  Duggan’s flowchart can help make these decisions.  About his flowchart, Duggan says that “it does not cover the plethora of data or map license types, this chart provides an easy reference as to whether you may or may not use the material you intend to use. Of course this may vary from country to country and on a case by case basis; also this does not serve as a legal document; legal advice should be obtained in case of dispute.”  I find the flowchart to be very useful and applaud Mr Duggan for creating and sharing it.

And yes, following his own advice and ours that we wrote about in the book, I did ask his permission if we could refer to his resource in this blog!  Thank you Nicholas.

Nicholas Duggan's article and flowchart to help GIS users decide if they can use a data set.

Nicholas Duggan’s helpful article and flowchart to help GIS users decide if they can use a data set.