By: Lauren Blumberger
Geovis Course Assignment, SA8905, Fall 2015 (Dr. Rinner)
Are you ever troubled by the amount of large files that you need to download and store on your computer? If so, a Web Map Service (WMS) may be useful as it delivers georeferenced map images over the internet that are generated by a map server. All you need is an internet connection to be able to access maps from a remote server and load them into your GIS. A request is made, and the response is one or more map images (returned as PNG, JPEG etc.). While WMS services are commonly used for retrieving base maps, they also provide a quick and easy way to work with data that has already been symbolized and styled by the author. Many WMS servers also have time support, WMS-T, which enables users to define a temporal subset for the rendering of layers that are properly configured with a time dimension.
In this post, I explore how to work with WMS-T retrieved maps in QGIS and how to animate them using Anita Graser’s Time Manager plugin (http://anitagraser.com/projects/time-manager/ and https://plugins.qgis.org/plugins/timemanager/). Anita Graser’s Blog includes a guest post by Karolina Alexiou on using Time Manager for WMS-T layers. I will describe the process, as well as discuss the limitations.
The first step is to find the service layers that you want to work with. This is not always the easiest task as you must use the GetCapabilities URL to access the maps, which in some cases doesn’t reveal the image until the connection is made from QGIS to the server. Thus, there might be some trial and error in terms of retrieving the maps you are looking for.
The map I use for this demonstration is made available by the Fire Information Resource Management System (FIRMS), who offer fire-based map images. The WMS-T allows access to FIRMS Active Fire Hotspots for a specific day. The GetCapabilities URL is provided on the website:
https://firms.modaps.eosdis.nasa.gov/wms-t/?SERVICE=WMS&VERSION=1.1.1&REQUEST=GETCAPABILITIES
Navigate to Add WMS/WMTS Layer in QGIS and click on new. Add the above URL and name the connection FIRMS. Make sure to enable the Ignore GetMap/GetTile URI reported in capabilities, as this will redirect you to the WMS instead of the WMS-T. When you click connect, multiple layers will appear. Load the MODIS_Hotspots layer to the canvas.*
Now that you have connected to the server you can overlay this image with other layers accessed through servers or available locally. Download any country boundary shapefile and load it to the canvas. Now you can change the style of this new layer and adjust the transparency to make sure that you can see the MODIS image underneath.
The next step involves the TimeManager plugin, which provides some support for stepping through this spatial temporal data.** Once you have installed the plugin, click on settings and add raster layer. The WMS-T standard uses many different time formats and the plugin requires you to know this format for inputting the start and end times. Therefore, you must examine the XML document with the web service description and find the section that defines the format of the time dimension. For this example, add the MODIS layer and input start time as 2005-11-16 and end time as 2015-11-16. Set the time step to months and press play. Now you will see that TimeManager renders each month interval by querying the FIRMS WMS-T.
Since WMS-T is a live service that will automatically update on the map, there are significant delays when working with these layers. Querying the web service and waiting for response takes time, and thus, the interactive mode using TimeManager does not result in a visually pleasing animation. TimeManager includes an option to export all the frames, which you can then move into another software to create a smooth animation.
The resulting animation shows the MODIS Active Fire Hotspots globally for the past ten years. The visualization works best if you maximize the video to full screen view.
Overall, working with WMS-T servers can be tricky. As there are many different time formats that can be used, it is quite a complex task to support them all in a GIS. Also, while you want to be connected to the server to be able to access live, current data, this connection slows down the rendering process and creates choppy animations. This being said, exporting the frames to create an animation provides a nice result and a fast way to visualize current data. As FIRMS constantly provides access to active fire data with the MODIS satellite, through reconnecting to the WMS-T and adjusting the time I can update the animation to 3 hours ago.
*The MODIS satellite detects both flaming and smoldering fires ~1000m2
**Note that FIRMS WMS-T only supports one date per request and date ranges are not allowed. To make Time Manager send the correct request, the source code file wmstlayer.py must be adjusted. If Time Manager is installed, the file can be found in the user folder on your computer. Replace lines 59 to 63 with:
self.layer.dataProvider().setDataSourceUri(self.IGNORE_PREFIX + \
self.originalUri + self.addUrlMark() + “TIME={}” \
.format(
time_util.datetime_to_str(startTime, self.timeFormat)))
This ensures that Time Manager only sends a point in time. A time range will return a black image for this WMS-T.
Special thanks to Dr. Eric Vaz and Anita Graser for their help.