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Exercise Title: Viewing Web Mapping Service (WMS) Operational Product Images in IDV & Google Earth
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Abstract: This exercise covers the Web Mapping Service resource sites and how to view their data with 2 important browsers.
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Preliminary Reading (in OceanTeacher, unless otherwise indicated):
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Web Mapping Service (WMS) - "The OpenGIS® Web Map Service Interface Standard (WMS) provides a simple HTTP interface for requesting geo-registered map images from one or more distributed geospatial databases. A WMS request defines the geographic layer(s) and area of interest to be processed. The response to the request is one or more geo-registered map images (returned as JPEG, PNG, etc) that can be displayed in a browser application." [From the OGC Standards List]
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Required Software:
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Other Resources:
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Author: Murray Brown
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Version: 8-6-13
"The maps returned by a WMS service are images only. They do not contain actual data." [ESRI website]
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1. The relatively new OGC services that can be applied to the problems of presenting data (WMS, WFS, WCS, etc) are wonderful tools. But the ultimate users don't see everything going on behind the curtain. I hope this exercise will reveal some of these secrets:
- As stated above, WMS only delivers images of data, not data. You can look, but -- essentially -- you can't touch.
- Other OGC services can deliver data:
- Web Feature Service (WFS) optionally delivers the actual vector data (points, lines, polygons, etc.)
- Web Coverage Service (WCS) optionally delivers grids
- "Optionally" means it's technically feasibly, but possibly not enabled by the publisher
- WMS and the other services are fundamental parts of online, dynamic ATLASES. You cannot see this, due to the finely written shells around the basic OGC services.
- All WMS work through browsers, not stand-alone. Good browsers include IDV and Google Earth and several ESRI programs.
- WMS server locations, i.e. the URLs, are often incomplete. Sometimes they lack the final part of the complete URL string: ?service=wms&request=getcapabilities. This is the minimal termination, but there are additional optional arguments.
- Even when successfully visualized, WMS images must be "refreshed" every time there is a change in physical size or placement. So don't expect the smooth globe spinning in Google Earth.
- About half the WMS services cited are not in operation, or they require additional URL arguments that you don't know and probably can't find.
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| 2. Run IDV. |
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3. If you are not looking at your area of interest, then select PROJECTIONS > PREDEFINED > LIBERIA (for example).
NOTE: This is needed at the beginning to reduce the size of the first requested image. A global high-resolution SST image could take all day, so don't omit this step.
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| 4. Now you see your area. WMS image requests will automatically be limited to this area. |
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5. In IDV select DATA CHOOSERS > CATALOGS and insert the WMS URL for ETOPO1 from the MDL WMS list below (marked in red). It' not really an "operational" product, but it's good looking and well illustrates the method.
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| 6. Select SHADED RELIEF and click on ADD SOURCE. |
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7. Now the WMS CONTROL appears, which means that IDV is ready to make and send a formatted request to the WMS server:
- Specific layer (based on choice in above Panel)
- Specific area (based on PROJECTION)
- Image size (based on current IDV setup)
- Format (usually gif)
- Etc.
Click on CREATE DISPLAY to send the message.
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| 8. The image is sent back from the WMS server and displayed in IDV. Notice how the shading makes it easy to interpret. |
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| 9. You can save this with FILE > SAVE AS and navigate to PRODUCTS > IDV > XIDV and use the name relief_image_liberia_etopo1_ngdc_wms.xidv |
| 10. Also, if you just want to save the above image, then select MAP > VIEW > CAPTURE > IMAGE. |
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| 11. Now we'll try some of the WMS servers in the MyOcean WMS list below. |
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12. Using the same steps you see above, here we're viewing the contents of the GLOBAL Ocean, Ocean Optics Products (daily observation) item in the list (marked in red).
In this case, you can expend the menu below the parameter level, to find this list. Each item is a different color palette. You can select one, as I have done here.
Then click ADD SOURCE.
NOTE: Generally, I can't find any difference between the palettes, and usually just take the default by not making any palette choice. I'm not sure that the palettes have been applied an many cases.
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| 13. And here is the image returned by the WMS. There isn't much to see, except a band of slightly higher values along the coast. There is a LEGEND, but it's hard to see. |
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14. Click on the LEGEND to enlarge it. Now you can see that the "remote sensing reflectance" is about 2500. I'll leave the units and definitions to you.
Presumably higher values represent particle-filled waters, due to light-scattering, which seems reasonable from this figure.
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| 15. And just for your interest, here's the same dataset, viewed with WORLD projection. You can see the large central-ocean oligotrophic areas, and the coastal area with higher values. |
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16. This WMS from the MyOcean list was next explored: GLOBAL Ocean OSTIA Sea Surface Temperature and Sea Ice Analysis (marked in red)
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No result on 20130806-7, regional or global. Returns a LEGEND but no SST map. |
17. And then this WMS from the MyOcean list was explored: GLOBAL Ocean Biogeochemistry Analysis >Nitrate (marked in red).
The use of a global palette, the usual practice with WMS products, results in a very indistinct image for Liberia. This is a universal problem with WMSs and atlases in general, the difficult balancing act between large-scale and small-scale areas, and the choice of palettes.
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| 18. Here is the global image for nitrate, where the palette looks perfectly good due to the wide range of values actually encountered. |
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19. But if you expand the LEGEND for the nitrate, you get this odd figure, not the expected color-values matchup.
It's Munch's "The Scream" in GeoTIF unsuccessfully displayed by a standard TIF viewer. This very small joke may originate in the WMS or in IDV.
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| 20. Now we're going on to see how to view WMS image in Google Earth (GE) |
| 21. Run GE. |
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22. We're going to add this item from the MDL WMS list (marked in red).
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| 23. Select ADD > IMAGE OVERLAY |
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24. Enter a NAME to identify the overlay. Then click on REFRESH.
NOTE: The LINK space is not where you put the URL for the WMS site. Google Earth will create a much longer, complex URL for the same site, using your "WMS Parameters". You can see the final long URL below in Panel xx.
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| 25. On this tab you can make settings to control how often GE checks back to the WMS for updates. But we want only to select WMS PARAMETERS. |
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26. This is the screen to select the map objects from the WMS. First we need to add in the new WMS. Select ADD.
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27. Into this small screen insert the URL for the JCOMMOPS SOOP site. Then click OK.
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28. Now GE interrogates the WMS and gets a list of the objects that can be mapped. Examine it carefully.
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29. Select the items you want to map, and move them to the right side with ADD->. Then click OK.
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| 30. And here the objects appear on the GE map. You can explore all the visualization options in GE, including turning ON or OFF different choices from source WMS. All this fun is up to you. |
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31. For your information, if you watched the contents of the LINK space in Panel 24 above, GE automatically filled it with items that reflected your choices. Here the "link" is broken into parts so you can see how it is structured:
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32. The above panels illustrate only the main steps in visualizing images from WMS resource. Obviously there are hundreds of details you can explore and refine on your own. Please inform this author of newer and better things you discover.
One topic not covered above is the actual dates associated with individual analyses. WMS is an ideal tool for display of climatological products, where temporal concerns can easily be addressed in the data object name, e.g. Average SST for All January Data. But automation of near-real-time products has left dates somewhat in the dust. If WMS will continue to be popular, then the end user's control (or at least knowledge) of specific dates must be augmented.
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