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We often hear from users that they have data that they would like to add to their Collect Earth Online (CEO) projects. Fortunately, CEO provides two easy ways to add your own imagery to your projects. The first is by connecting your own imagery via Web Map Service (WMS) or Web Map Tile Service (WMTS), and the second is by connecting your imagery hosted in Google Earth Engine (GEE). In this blog post, we will focus on connecting WMS/WMTS data sources to CEO.
WMS and WMTS provide specifications for serving georeferenced maps over the internet. Many national datasets are available through this protocol.
You may be familiar with using WMS or WMTS imagery in desktop applications such as ArcGIS or QGIS. Like these desktop applications, CEO can display WMS and WMTS imagery though the institutional imagery feature.
The first step of preparing to connect to your WMS/WMTS with CEO is checking that your WMS/WMTS is compatible with CEO. CEO requests data in EPSG:3857. Your WMS/WMTS server is responsible for reprojecting the imagery it hosts into this projection. Fortunately, most WMS/WMTS will do this automatically. Once your data is loaded, you can always visually check that they align with known good imagery, such as MapBox.
To connect your WMS/WMTS to CEO, you need to gather three pieces of information: the base URL for your WMS/WMTS, the layer name that you want, and any parameters your WMS/WMTS requires to serve imagery. If you need help gathering this information, there are more details provided in the next section.
The basic steps to connect your WMS or WMTS imagery are:
Once you’ve added the layer from the WMS/WMTS, you can add it to any of your projects and leverage the imagery during data collection.
However, sometimes users have difficulty identifying the correct URL, layer name, and parameters to use to connect their imagery successfully. Here’s some tips that we recommend to help you find this information.
First, find the information page on your WMS/WMTS. These websites will frequently contain the necessary information to connect your WMS/WMTS to CEO. Sometimes your desired data source will have a more complex information page, with information about the data and then links to multiple WMS/WMTS. Some examples include:
When looking at this information, the first piece of information to look for is to locate the WMS URL. For example, https://geos1.snitcr.go.cr/Ortofoto2017/wms?VERSION=1.1.1.
An important thing to note is that everything after the ? in your WMS/WMTS URL is a parameter. When adding it to CEO, it should not be considered part of the URL. However, you will want to include the ? when you add your URL to CEO–but don’t worry, we’ll add it automatically if you forget!
The second piece of information to look for is the layer names that are available. The information page will frequently contain a list of layers that can be viewed with the WMS/WMTS. Note that sometimes your layer may have multiple names, so through the web interface you must make sure you are viewing the correct name.
You can also connect the WMS/WMTS into a desktop application like ArcGIS or QGIS and access the layer names through the desktop application.
You can then inspect the Layer Properties in order to find the information you need to connect the WMS/WMTS layer to CEO.
If you cannot load data into a desktop GIS and your WMS/WMTS only uses an XML file that does not have any style information associated with it, you can still find the layer name but it is more difficult. In this case, the information is in a difficult to read document tree. If this is the case, and you can’t load the data into a desktop GIS, search for “Layer” and look for the text <Layer> with <Name>, <Title> and <Abstract> information fields that follow it. The <Name> text will be what you are looking for. In this example, the name is ‘GEBCO_2020_Grid_3’.
The final piece is to identify the parameters. In our URLs, everything after the ? is a parameter, separated by the “&” character. Common things that you may see in URLs include:
We do not want to include any of these three in our CEO parameters. This is because
Other parameters may be necessary. For example, some WMS/WMTS have a “map” parameter that is necessary. Other parameters you might see include “crs” (generally not needed), “format” (also generally not needed). We suggest reading additional resources on the WMS GetMap function to help you to identify any additional parameters your WMS requires to serve imagery.
If you do load your WMS in a desktop GIS, you can use the Layer Properties interface to help you find the appropriate properties to include.
Now that we have the crucial information, we can connect the WMS to CEO.
If after you try these steps you are still having trouble, here are some additional troubleshooting hints.
Note that this server requires SRS instead of CRS. Both are OGC compliant. While you’re creating the working WMS URL, you might get the following error:
Constructing these can be difficult. One of the best ways to construct them is to use your browsers’ console or developer tools, looking at the “Network” pane. When you visit your project in CEO and try to load the data, it should populate with multiple requests for the different map images. You can double click on these in order to visit the individual image. If this doesn’t work, you can try adding or changing the URL until an image appears.
Clicking on one of these (e.g. https://www.gebco.net/data_and_products/gebco_web_services/2020/mapserv?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&FORMAT=image%2Fpng&TRANSPARENT=true&LAYERS=GEBCO_2020_Grid_3&WIDTH=256&HEIGHT=256&CRS=EPSG%3A3857&STYLES=&BBOX=-8766409.899970295%2C-313086.06785608083%2C-8609866.866042254%2C-156543.03392803986) will attempt to show a single image.
If you haven’t configured the WMS correctly, clicking on one of these requests gives an error message. For example, here’s an example error:
<?xml version=’1.0′ encoding=”ISO-8859-1″ standalone=”no” ?>
<!DOCTYPE ServiceExceptionReport SYSTEM “http://schemas.opengis.net/wms/1.1.1/exception_1_1_1.dtd”>
<ServiceExceptionReport version=”1.1.1″>
<ServiceException code=”MissingParameterValue”>
msWMSLoadGetMapParams(): WMS server error. Missing required parameter SRS
</ServiceException>
</ServiceExceptionReport>
For this WMS, getting rid of the version=1.1.1 parameter resolved the configuration issue.
We suggest reading resources on the WMS GetMap function to help you interpret these errors, for example: http://opengeospatial.github.io/e-learning/wms/text/operations.html#getmap
CEO would like to thank its ongoing funders FAO, NASA–USAID SERVIR, and SilvaCarbon, a US government program. Thanks also to CEO’s technology partners: Norway’s International Climate & Forests Initiative for funding open high-resolution data availability; Planet for providing high-resolution imagery; and the Google Earth Engine team for creating a platform for Earth science data and analysis.
Collect Earth Online is working constantly to improve the user experience, and your feedback is invaluable. If you have ideas to share, please write to support@collect.earth.
Thank you!
