One of the new features in the Surfer mapping software that hasn’t gotten a lot of mention is the ability to download imagery as grids. Most of you may know that Surfer can connect directly to WMS servers and download georeferenced images directly into your maps as base layers. You can download aerial photographs, orthoimagery, shaded topographic images, scanned USGS maps, images showing geological bedrock, etc. However, did you know you can also download imagery as grid files and use them to create contours or other grid-based maps?
You can download images as grids from WMS servers using either the Grid | Grid from Server command, or by clicking the Download button in the Open Grid dialog when creating a grid-based map. The difference is that the Grid | Grid from Server command downloads the image as a grid file directly to your hard drive so you can use it later on (e.g. for blanking) or in multiple maps.
Download images from WMS servers and use them directly in Surfer as grid files.
For example, let’s say you have a base map of the major Hawaiian Islands and wish to add a visual aspect of the elevation (but don’t have or need exact data). You can:
1. Load the base map. Click Map | New | Base Map, select HI_Outline.bln from our website click Open.
A base map of the major Hawaiian Islands is created
2. Now let’s download a terrain image as a grid file, blank it to the outline of the major Hawaiian Islands and add it to the map. The first step is to download the grid.
a. Click Grid | Grid from Server. The Download Online Grids dialog opens.
b. Select a server and a layer of terrain data. For example, expand the Terrain Data category and expand the USGS IMS Ref server.
c. Select the 1/3 NED Hawaii Shaded Relief layer. When the layer is selected, you can see a preview of the image.
d. The number of pixels in the image will determine the grid resolution. If you want a higher grid resolution, expand the Select Grid Resolution to Download section and move the slider to the right to the desired resolution.
e. Click OK.
f. In the Save Grid As dialog, choose to save the grid file and click Save.
g. In the Export Options, check GS Reference (Version 2) file to save the spatial reference information, and click OK.
3. Now the grid has been saved. Let’s blank it so that there is no image data outside the island boundaries.
a. Click Grid | Blank.
b. Select the grid file and click Open.
c. Select the BLN file we initially imported and click Open.
d. Give the new grid file a name and click Save.
e. In the Export Options, check GS Reference (Version 2) file to save the spatial reference information, and click OK.
f. Click OK in the message that says the blanked grid file was created.
4. Now we can add the image map.
a. Click on the existing map to select it and click Map | Add | Image Layer.
b. Select the blanked grid and click Open. If you get a warning that the new layer exceeds the current map limits, just click No.
c. Select the new Image layer in the Object Manager, and in the Property Manager, on the General page:
i. Under General, you can click the (…) button to the right of colors, load a CLR file (download here), and click OK.
ii. Under Hill Shading, make sure Enable hill shading is not checked.
iii. Under Missing Data, set the color to Baby Blue and the Opacity to 30%.
Add the grid file downloaded from the WMS server as an image layer.
5. Now you can convert the coordinate system of the map and add other map data, such as a map scale bar, graticule, or other map layers.
Complete the map by converting the coordinate system of the map or adding other map data, such as a graticule or map scale.
When you add the color scale bar, you will notice that the Z values for the Image layer range from 0 to 1.
The Z values of the grids from the downloaded images range from 0 to 1 instead of having true Z values.
This is because WMS servers do not provide imagery with Z values. Because these are images without Z values, Surfer's mapping software will convert the color of the pixel in the image to a value between 0 and 1. Therefore, these maps do not show true elevation. The downloaded grids are still useful though, as you can get an idea for the relative elevation of the area and display it in an informative fashion. As one Surfer 13 user says: “I like downloading grid files online, even if it’s just seeing relative (0 to 1) elevations it’s helpful.”
One of the main sources of frustration when downloading online maps or grids is finding a WMS server with imagery for your area of the world. We have compiled a list of URLs for working WMS servers on our website. However note that when you download online images as grids, you want to make sure that the color in the image reflects the variable that you want the Z value to represent. For example, aerial photographs would not be a good choice for downloading as a grid because the colors in the image could represent trees, buildings, roads, water, etc. The colors in the image do not reflect the elevation or any other value. For elevation values, I recommend using a server showing a terrain image in grayscale color. For example, in the Download Online Grids dialog, under the Terrain Data category there are a few servers of terrain data for the United States.
It would be great if Surfer could download the grid-based data so that the grid would have real world Z values. That would require support for other server types (such as WCS servers), which more commonly offer images with true Z values associated with them. Look for this in a future version!