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Posts Tagged ‘ArcGIS Online’

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.

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US Congress: Maps and Data Portals

Did you know that the US Congress is increasing its use of web mapping tools in GIS?  In one of these map collections from the US Congress, you can discover organizations who host maps and data.  The themes of these maps include business locations, wildfires, hurricanes and current weather, public transportation, education, and many others.  For education, these maps could be useful for discussions in political science, geography, biology, business, and in other disciplines, as well as access to data for courses that use GIS.  To find out why the US Congress finds value in web mapping and GIS, read the excellent article that my colleague Lauren Lipovic recently wrote about how GIS is increasingly being used to inform and disseminate public policy.   Also, see the government policy page from Esri for more case studies, maps, and data.

The Mapping for Congress Featured Set contains some maps on the above themes; the most compelling one in my opinion is the set for Senator Risch from Idaho, on wildland fire potential, brownfields development success stories, and other themes of interest to citizens of that state and to those living elsewhere.  Another notable example from the Featured Set collection is this new map of Virginia state parks.

The Federal Agency Map Collection assembled by my colleague provides another excellent way of accessing more of these maps and data sets.  This collection includes the Mapping for Congress Expanded Gallery.  While some of the maps are applications that do not allow for streaming or downloading the data, GIS users with a bit of digging will find plenty of layers that they can use for analysis.  And equally importantly, these data sets represent a trend that we noted in our book–with increasing use of web mapping tools such as Esri story maps and the ability to assemble these map galleries, decision makers are increasing their use of GIS and spatial analysis in their decision making, which is good news for all of us.

id_map_gallery.JPG

The Idaho Map Gallery.

Landsat Thematic Bands Web Mapping Application in ArcGIS Online

December 20, 2015 2 comments

Teaching remote sensing?  Or just want to understand remotely sensed imagery better?  The Landsat Thematic Bands web mapping application can serve as a very useful teaching, learning, and research tool.  It covers the entire planet and the map is updated daily with new Landsat 8 scenes.

You can access many band combinations and indices by hovering over the tools to the left of the map image and selecting among the following:

  • Agriculture: Highlights agriculture in bright green. Bands 6,5,2
  • Natural Color: Sharpened with 25m panchromatic band. Bands 4,3,2+8
  • Color Infrared: Healthy vegetation is bright red. Bands 5,4,3
  • SWIR (Short Wave Infrared): Highlights rock formations. Bands 7,6,4
  • Geology: Highlights geologic features. Bands 7,4,2
  • Bathymetric: Highlights underwater features. Bands 4,3,1
  • Panchromatic: Panchromatic image at 15m. Band 8
  • Vegetation Index: Normalized Difference Vegetation Index (NDVI). (Band5-Band4)/(Band5+Band4)
  • Moisture Index: Normalized Difference Moisture Index (NDMI). (Band5-Band6)/(Band5+Band6)

The Time tool for different indices at larger scales based on a user-selected location enables examination of changes over years or over seasons.  It also provides temporal profiles for NDVI (Normalized Difference Vegetation Index), NDMI (Normalized Difference Moisture Index) and an Urban Index, dating back to 1973.  The Identify tool enables access to information on the date, cloud cover, and a spectral profile about each scene.  The Bookmark tool allows access to interesting locations such as the “Eye of the Sahara” in Mauritania.

The application is written using Web AppBuilder for ArcGIS accessing image services using the ArcGIS API for JavaScript, with access to the following Image Services:

  • Landsat Multispectral on AWS – 8-band multispectral 30m resolution image services and functions that provide different band combinations and indices.
  • Landsat Pan-sharpened on AWS – Panchromatic-sharpened imagery; 4-band (Red, Green, Blue and NIR); 30m resolution.
  • Landsat Panchromatic on AWS – Panchromatic imagery; 15m resolution.

These services can also be accessed through the public Landsat on AWS group on ArcGIS Online.  Because you can add these services as layers to your own maps or are adding to maps made by others, or if you are simply using the above web mapping application as a standalone map, you truly have “the world at your fingertips” with these maps and apps.  But there is a third option: Use the Unlock Earth’s Secrets page, also useful for instruction, with the above application embedded in it, but also with explanatory text and featured places around the planet as they have changed through time.

Think of the above as solid introductory segments to help your students, customers, or stakeholders see the value in remote sensing.  These maps and applications require very little geospatial technology skills to use, but allow you to focus on building remote sensing concepts and principles while exploring some truly engaging content and places.

To dig deeper, delve into the many powerful remote sensing functions available in ArcGIS Desktop.  One source for engaging, hands-on activities, is Kathryn Keranen and Bob Kolvoord’s book Making Spatial Decisions Using GIS and Remote Sensing:  A Workbook.

Give these resources a try!

Landsat web application

Landsat web application in ArcGIS Online.

The National Geologic Map Database Map Viewer

December 2, 2012 1 comment

As we describe in the book The GIS Guide to Public Domain Data, the AASG-USGS National Geologic Map Database (NGMDB), is one of the most useful resources to access geologic maps and data.  The NGMDB, through its Geoscience Map Catalog, provides access to  more than 89,000 maps and reports by  more 630 publishers.  More than 23,000 of these publications are geologic maps.  A significant leap forward since the book was published is the new NGMDB Map Viewer.  This Mapviewer is built using Esri ArcGIS Server and the Image Server Extension. With the viewer, inside ArcGIS Online inside an ordinary web browser, data users can access a significant component of the national archive of the systematic, regional mapping of  bedrock and surficial deposits, conducted by Federal, State, University, and private entities, stretching back to the 1800s.

Why access the NGMDB through this map viewer?  There are several advantages to doing so:  You can view numerous maps in the NGMDB Map Catalog in one seamless interface running in your web browser.  You can toggle between selected map types, search by keyword to find a record in the Map Catalog, find related publications, and best of all, access the Map Catalog to download high resolution geotiffs and other GIS-compatible formats.  I formerly worked at the USGS, and have been in touch with colleagues there over the past year, eagerly awaiting the arrival of this map viewer.  If you have read our book or have been reading this blog, you know that Jill Clark and I cast a rather critical eye on any data portal or viewer, assessing whether any of them are truly useful for the GIS data user.  I am pleased to report that after testing the NGMDB Map Viewer, it met all of my expectations.  True, the database is largely (but not entirely) US-based, but the interface is easy to use and allows the data users to obtain what they came for–GIS-compatible geologic data of different areas, scales, and themes.

To access the database, go to ArcGIS Online:  http://www.arcgis.com/home and search for “USGS NGMDB Map Viewer 2012”, or directly to http://ngmdb.usgs.gov/maps/mapview.

National Geologic Maps Database Viewer in ArcGIS Online

National Geologic Maps Database Viewer in ArcGIS Online