Using a Web Map Service (WMS) is a great way to easily get up-to-date imagery for your project. An issue you may run into, however, is that a server may not allow you to download images over a certain size (in pixels). The image resolution provided on most servers is usually high enough for most projects but there may be some instances where a higher resolution is needed (e.g. a poster-sized map or a web map that covers a large area).
One possible workaround for this issue is to get several small-area, high-resolution images from a WMS and then mosaic them together to make one large-area, high-resolution image. Here, we will use the City of Toronto Web Map Service to:
Get an image of downtown Toronto
Tile the image to split it into four smaller images
Use the extents of the four tiled images to get four high-resolution images
Mosaic the images to one large high-resolution image
Download an Image from the Web Map Service
First we will get an image of downtown Toronto from the City of Toronto WMS. Click the Advanced Import button on the Geographic Imager panel. Select Web Map Service from the Format drop-down and click Browse. Click Load services from Avenza and select City of Toronto WMS from the list.
Download an Image from the Web Map Service (Click for larger version)
Select City of Toronto Imagery from the list of layers. Click Select Area then drag a box to zoom in on an area of downtown. Click OK to return to the previous window. There is no need to change the image size because we will use this image to get the extents of a higher resolution image. Leave the other options as default and click OK to load the image, and click OK again on the Advanced Import dialog box. The selected image will now open in Adobe Photoshop.
Tile the Image
Next, let’s tile the image to split it into four separate images. Open the Tile dialog box from the Geographic Imager panel. Choose By Number of Tiles as the Tiling Schema and change Horizontal and Vertical to 2. Change the Horizontal and Vertical Overlap to 3 percent. It’s important to have overlap between the images so they will mosaic properly when the data is transformed.
Tile the Image (Click for larger version)
Click the Keep Images Open check box to enable it. Choose a name and location to save the tiles. You can save them to a temporary location because they will not be part of the final product. The four image tiles will open in separate tabs.
Estimate a Web Map Service’s Maximum Image Size
Open the City of Toronto Imagery WMS again and select an area. In this service, as with many other services, the maximum resolution is not provided. We can, however, use trial and error to find the largest image that the service will allow us to download. A WMS has a maximum allowed width and height set for an image request. The maximum width is usually the same as the maximum height.
Set the resolution of the image by adjusting the width of the image in pixels under Output Options. First, try setting Image Size to a pixel width of 5000. Height will update automatically based on the image’s dimensions. Click OK and you will see an error that says “Parameter ‘width’ contains unacceptable value” (or height if the images higher than it is wide). OK the error to close it.
Estimate a Web Map Service’s Maximum Image Size (Click for larger version)
To estimate the maximum width and height allowed by a web service, you can adjust the image size to see when the server returns an error. The maximum width and height for the City of Toronto WMS is about 4000 pixels.
Downloading and Mosaicking Large Images
Next we will use the geographic extents of the four tiled images to download four higher resolution images with matching extents then mosaic the images together. Import an image again from the City of Toronto WMS using Advanced Import. Select an area, click Select Area by Another Document’s Extents and choose the first tiled image from the dropdown menu. The Select Area dialog will display an area matching that image’s area. Click OK to return to the previous screen.
Downloading large images (Click for larger version)
Enter 3000 as the Image Size. Click OK to add the image to the Advanced Import dialog box. Repeat the above steps for each of the remaining three tiled images.
Creating a mosaic of four images (two by two) makes a single image just under 6000 pixels wide because of the overlap between the tiles (the height will vary depending on the area you selected). This is larger than the maximum size allowed by the WMS. Check the box Mosaic All Files to the Destination Document and select one of the images currently loaded in the dialog window from the drop-down menu. Leave the other options unchecked and select Normal as the Layer Blending Mode. This will merge the four images into a single document.
Mosaicking large images (Click for larger version)
Avoid Downloading Images Greater than the Maximum Image Resolution
One further consideration when getting imagery from a service is to avoid requesting an image that is higher resolution than the full resolution of the image on the service. In this case, the number of pixels in the image and, therefore, the image size will increase without any increase in the actual resolution of the image.
To see an example of this, get an image from a WMS by zooming in on a small area and downloading images of varying resolution. The pictures below show two images of the same area side by side at different resolutions. The image on the left is 1024 by 870 pixels and the one on the right is 2500 by 2124 pixels but there is no noticeable difference between them. This is because the image exceeds the resolution of the image on the server so the WMS resamples the image to a higher resolution, creating duplicate pixels.
Avoid Downloading Images Greater than the Maximum Image Resolution (Click for larger version)
With the latest release of Geographic Imager 5.2, it’s now possible to easily import images directly from an ArcGIS Online account or an ArcGIS web service. This will allow you to use shared data within your ArcGIS Online organizational account and connect to publicly available map servers from various online sources.
ArcGIS Online is a collaborative web GIS that allows you to store and share GIS data using Esri’s secure cloud. Before, you may have had to download raster layers to your local machine and then import them into Adobe Photoshop using Geographic Imager. Now, Geographic Imager has a much-improved workflow to get ArcGIS Online image layers into Adobe Photoshop with full georeferencing.
Currently, the types of datasets allowed are Map Image Layers and Tile Layers. To load a layer, open Advanced Import and select ArcGIS Online from the Format drop-down list. Enter the credentials for your ArcGIS Online account and select an image layer from your user portal.
Images can be resized and transformed on import. To extract a specific area from the image, click Select Area. The interface is the same as the one used for WMS Import.
In addition to using your own organization’s data, you can connect to publicly available data from a wide variety of organizations by connecting to an ArcGIS Web Service. To connect to a web service, use Advanced Import and select ArcGIS Web Service from the Format drop-down menu. Click Browse and enter the URL for the service. This is a great option when searching for data from open data portals created by government agencies.
Avenza Maps is capable of importing and displaying georeferenced maps in several different formats. The app processes a map by rasterizing and tiling it on import. The speed and result of processing will vary depending on the type of file used and the size and dimensions. We’ll discuss the types of files allowed and the advantages of each.
Usually larger files take longer to process than smaller ones with some exceptions. An image may have a very high resolution but a small disk size due to image compression. It is recommended that images be less than 100 megapixels for upload to the app. A geospatial PDF file with many vector features may also take a long time to process. In this case, you should convert the file to an image to avoid having to rasterize vector data in the app.
Maps can be loaded into the app faster by processing them before import. Maps can also be preprocessed by uploading them to the Avenza Maps Store or using the Export to Avenza Maps tool in Geographic Imager or MAPublisher.
Comparison of Allowed File Types
GeoTIFF
A GeoTIFF is a TIFF file (Tagged Image File Format) with spatial information embedded in the file. This format is typically the preferred format for uploading directly to Avenza Maps without preprocessing. Since this is an image format, the app doesn’t need to rasterize the file on import meaning processing times will generally be faster compared to PDF’s, and it will maintain the native resolution.
It is also possible to import a normal TIFF file by zipping it with a TIFF World File (TFW) and either a WKT or PRJ file which contains projection information. It is best not to do this for the sake of simplicity. Usually, any GIS software that can produce a world file will also be able to save to GeoTIFF.
Geospatial PDF
A Geospatial PDF stores spatial data in either raster or vector format as well as associated attributes and layer information. Files should comply with the Adobe Acrobat PDF 1.7 specification which has been standard since 2006.
Avenza Maps rasterizes geospatial PDF files on import which discards layer and attribute information. PDF files typically take longer to process than image files. There is also a chance that the file may process incorrectly (e.g. discarding labels).
Despite this, there are some instances when it may be preferable to use a geospatial PDF file. For example, the map may already be in this format and isn’t worthwhile to convert to another format. Geospatial PDF files tend to be smaller than image files so they would be a good solution if device storage space or download speed is an issue. Also, occasionally converting from PDF to TIFF degrades the image quality and may make maps illegible.
To ensure a geospatial PDF file has a good balance between size and quality, open the file in Adobe Acrobat and go to Advanced > PDF Optimizer. This is especially helpful if the map contains large raster images.
JPEG
JPEG is an image format like TIFF but it does not have the ability to store spatial information. To load a JPEG into Avenza Maps, it must be zipped with a JPEG World File (JGW) and either WKT or PRJ file. JPEG files are compressed so this may be an option to consider if file size is a consideration, however, a better alternative would be to compress a GeoTIFF file.
Processing Maps with MAPublisher, Geographic Imager, and the Map Store
It is best to process maps before loading them into the app because desktop computers are more powerful than mobile devices. It also avoids having to process the file each time the map is loaded onto a device. Geographic Imager and MAPublisher each have a utility to export to an Avenza Maps package with the capability to upload directly to the Avenza Map Store if you have a vendor account.
Running these utilities produces a folder that contains a ZIP file with a thumbnail to show a preview of the map, a reference file, and a folder called tiles that contains a tiled version of the map in PNG format. This ZIP file can be imported into the app in the same way as other files.
In the latest GeoJSON specification (2016), the coordinate reference system for all coordinates is a geographic coordinate reference system—using the World Geodetic System 1984 (WGS 84) datum—with longitude and latitude units of decimal degrees. The previous specification (2008) allowed for the use of alternative coordinates systems, but this was removed because of interoperability issues.
MAPublisher still recognizes GeoJSON files with a specified coordinate system even though it is no longer officially supported. However, if no coordinate system is specified, MAPublisher will assume the coordinates are in WGS 84. Occasionally, this may cause a problem of improperly formatted files that contain projected coordinates but have no specified coordinate system. In this case, users will need to either choose a coordinate system during import or modify the GeoJSON file by adding a coordinate reference system (CRS) object manually.
Selecting a Coordinate System on Import
To change the coordinate system using the Import dialog box, click Advanced and select the WGS 84 link under Coordinate System. Ignore the warning about changing the coordinate system by clicking “Replace coordinate system”. Select the correct projected coordinate system from the list.
Modifying the GeoJSON File Manually
Coordinate reference systems can be specified in a GeoJSON file using a CRS object. You can view the contents of any GeoJSON file by opening it in a text editor such as Notepad. Copy and paste the text below after the line: “type”: “FeatureCollection”, (usually on line 2). Change the EPSG number to the correct CRS for your dataset. See Spatialreference.org to lookup an EPSG code.
The latest release of MAPublisher includes the ability to trim and extend objects to a crossing or intersecting path. Extending a path lengthens it to meet the edge of a crossing object and trimming a path cuts the portion that extends past the edge of an intersecting path. Trim and extend tools are commonly used in CAD software and will greatly improve the ability to produce accurate and precise data in MAPublisher and clean-up imported data.
Location of the Trim and Extend tools on the Adobe Illustrator Tools panel. Click and hold the tool button to switch between MAP Trim Tool and MAP Extend Tool.
The MAP Trim Tool and MAP Extend Tools are located on the Adobe Illustrator Tools panel. Click and hold the MAP Trim or Map Extend tool button icon to switch between them. To use the tools, select the crossing or intersecting path and click the object to extend or trim. The diagram below illustrates the basic process.
Visualization of the trim and extend processes
There are many possible applications for these tools in digitizing and cleaning map data. In a hypothetical example illustrated below, we trim all the roads that extend past the edge of a border, and extend the imported roads (in bold) to meet the existing roads.
Examples of uses for the trim and extend tools. Extending imported roads to match existing roads and trimming lines that fall outside of a map borderLines trimmed and extended!
There are two methods of exporting GPS tracks from Avenza Maps in KML format: as a line and as a track. Which option to select depends on the intended purpose. Lines are much simpler than tracks and store only the position of each vertex in the line as latitude longitude and elevation. Tracks contain a complete description of how the path moves through space including the position at one-second intervals, the time at which each position was recorded, the compass angle of the heading, and the velocity. Lines take up much less storage space than tracks. To switch between exporting to Lines or Tracks, select Track Export in the Export Settings screen.
Export Settings screen
Tracks are useful in applications where the GPS position at a given time and space is important. For example, in Google Earth there is an option to record a tour that moves to predetermined places on the globe. This would be useful if you wanted to follow along on a hiking path or do a virtual walkthrough of a proposed building development. To make a tour from a track, open the KML file in Google Earth, select the track in the places panel, and click the Play Tour icon. The animation below shows a walk through Mount Pleasant Cemetery in Toronto, Canada. Lines contain only the location information: latitude longitude and elevation. Export to line when the time data is not important such as if you are making a map of a hiking trail.
Play tour in Google EarthA tour of Mount Pleasant Cemetery in Toronto, Canada using Google Earth
KML Code for Lines and Tracks
KML (short for Keyhole Markup Language) is an XML notation for displaying spatial data in web applications. It was developed for use with Google Earth and can be read by many programs. Avenza Maps uses KML to import and export placemarks, lines, and tracks. You can view and modify KML files in any text editor. The KML element used for lines is called a Linestring which is defined as “a connected set of line segments”. A Linestring element contains a coordinates tag which is a list of longitude and latitude positions in decimal degrees and elevations in meters.
KML Linestring code sample
Tracks use the KML element “gx:Track” which contains several tags.
when – the date and time a point was recorded in UTC
gx:coord – the location of the point in decimal degrees and the elevation in meters
gx:angle – the current heading in compass degrees (i.e. 0 degrees is north, 90 is degrees east and so on)
gx:value tag defined as “speed” – the current speed in meters per second
There are equal numbers of each of these tags. A full description of the vertex includes all the tags in the same sequence. For instance, the first when tag, coord tag, angle tag, and speed tag, describe the first vertex in the track.
With the latest release of MAPublisher 9.9, it’s now possible to easily import layers directly from an ArcGIS Online account or an ArcGIS web service. This will allow you to use shared data layers within your ArcGIS Online organizational account and connect to publicly available map servers from various online sources.
ArcGIS Online is a collaborative web GIS that allows you to store and share GIS data using Esri’s secure cloud. Before, you may have had to download layers as shapefiles to your local machine and then import them into Adobe Illustrator using MAPublisher. Now, MAPublisher has a much improved workflow to get ArcGIS Online layers into Adobe Illustrator will full georeferencing, all map features, and attributes.
Currently, the types of datasets allowed are Feature Layers, Map Image Layers and Tile Layers. To load a layer, use MAPublisher Import as you would with any data type and select ArcGIS Online from the Format drop-down menu. Click the login link to enter your ArcGIS Online credentials to access your organization’s web portal.
Import ArcGIS Online dialog boxArcGIS Online user portal
Feature Layers contain vector data that will import as artwork into Adobe Illustrator. Optionally, you can extract specific features using standard SQL queries. Map Image Layers and Tile Layers are raster data layers that can be added by selecting the geographic extents.
Import Image Map Layer
In addition to using your own organization’s data, you can connect to publicly available data from a wide variety of organizations by connecting to an ArcGIS Web Service. To connect to a web service, use MAPublisher Import and select ArcGIS Web Service from the Format drop-down menu. Click to select the dataset and enter the URL for the service. This is a great option when searching for data from open data portals created by government agencies.
Import ArcGIS Web ServiceValid ArcGIS Web Service
Accessing Esri’s online services through MAPublisher provides a great opportunity to use shared data within your organization and access a wide variety of publicly available data. We’re sure you’ll find it very useful for finding data to make great maps.
MAPublisher Grids and Graticules is a powerful tool. There are dozens of settings to create an indexed grid, measured grid, or graticule exactly the way you want. We often receive questions about how to create certain grid and graticule styles and this was interesting. We were asked how to create a graticule to display a very specific latitude and longitude, perhaps even by itself (only a single line of latitude or longitude).
Creating a single graticule line
In this example, we’re going to create a graticule that will only display the Tropic of Capricorn at -23.4371 degrees (or 23.4371 degrees south of the equator).
On the MAPublisher toolbar, click the Grids and Graticules button to open the dialog box. Click the Graticules button to create one. On the Graticules Grid main settings, the important setting to note here is the Pass through section — it specifies lines of latitude and longitude that must be included in the graticule. Enter 30 deg Long and -23.4371 deg Lat. This means that a graticule line must pass through -23.4371 degrees latitude (the Tropic of Capricorn). The reason why a line at 30 deg longitude is specified is to hide it from the map view (it is placed at 30 deg longitude outside of the map extent).
Pass through setting
In the Intervals section, set 90 deg Latitude interval and 180 deg Longitude interval. Because the intervals are at the extreme, this means that the the only lines left to display are the ones specified in the Pass through section (30 deg long and -23.4371 deg lat). In this case, it will only display a single line of latitude (the Tropic of Capricorn) for the graticule.
Intervals setting
If you want to label the graticule, go to the Line Labels setting. Click the Lat and Lng on the grid label control to enable them. To control how many decimals are displayed, click the Format setting, then change the number of decimals to something greater than 0.
Label settings
To intersect single lines of longitude and latitude, adjust the Pass through setting so that the line is within the map’s extent. In this case, it was set to -50 deg (50 deg west).
Intersect single lines of longitude and latitude
Remember that you can share Grids and Graticules settings with anybody by clicking the Save Settings button, selecting a destination folder and sharing the configuration files.
A graticule is the network of lines of latitude and longitude drawn at regular intervals on a map. Graticules are created in MAPublisher using the Grids and Graticules tool. In some maps, you may want to limit the area on the map that a graticule covers. For example, you may want it to cover only the map’s area of interest. The image below is a map of North America with a graticule drawn at 5-degree intervals. US State boundaries are drawn in white. In this post, we’ll modify the graticule three times so it conforms to the edges of the image, so it covers only the Continental United States, and lastly a combination of the previous two modifications.
North America with a graticule of 5 degree intervals.
MAPublisher can limit the geographic extents of a graticule in two ways: using Grid Bounds and using Grid Constraints. In both cases, you’ll specify the lower left and upper right corners of the graticule. Specifying Grid Bounds will limit the extent of the graticule to a rectangular area while specifying Grid Constraints will limit the graticule along lines of latitude and longitude. If both Grid Bounds and Grid Constraints are specified, the graticule will cover an intersection of the two areas. The image below shows bounds and the constraints and the intersecting area which forms the graticule.
Bounds and the constraints and the intersecting area which forms the graticule.
To modify a graticule so that it conforms to the edges of the image, you’ll need to specify grid constraints. In the Grids and Graticules dialog box, click the Specify Grid Constraints check box and set the Lower Left and Upper Right corners to the corners of the image which are -127°, 7° and -50°, 65° respectively.
Specifying Grid Constraints.
To create a rectangular graticule covering only the lower 48 states, click the Specify Grid Bounds check box and set the Lower Left and Upper Right corners to the corners of that area. Tip: click the MAP World Locations drop-down arrow to choose the values for the lower left and upper right corners.
Specifying Grid Bounds.
When both Specify Grid Bounds and Specify Grid Constraints check boxes are both checked, the graticule will cover an intersection of each of the extents. For instance, in the map below, the northern extent follows the 49th parallel at the Canadian border, the western extent is at the edge of the image (127° west) and the south and east extents are the same as in the previous map.
Both Specify Grid Bounds and Specify Grid Constraints selected.
When georeferencing a map in Geographic Imager, there are two tools which can be used to check spatial accuracy: Validate and Show Image Extents Online. With Validate, click a point on the image and it will show the corresponding location on the web map service so that you can compare the difference between them. Show Image Extents Online will display a rectangle representing the spatial extent of the image on the web map.
The image below shows the Validate tool in action. Selecting the tool and clicking on the road intersection brings up the same intersection in the web map, displaying how accurate the georeferencing may be. It is good practice to test several known points on the image. Choose features that will be easy to identify on the web map such as road intersections, coastlines, buildings, and landmarks.
The Show Image Extents Online tool is shown in the image below. Use this tool to see the full area covered by the image. Note that the rectangle shown on the web map will include the non-map areas of the page (borders, legend, etc).
Currently, MAPublisher and Geographic Imager are NOT officially compatible with macOS Tahoe. To avoid any potential issues, please refrain from updating to macOS Tahoe at this time.
To create a rectangular graticule covering only the lower 48 states, click the Specify Grid Bounds check box and set the Lower Left and Upper Right corners to the corners of that area. Tip: click the 
When both Specify Grid Bounds and Specify Grid Constraints check boxes are both checked, the graticule will cover an intersection of each of the extents. For instance, in the map below, the northern extent follows the 49th parallel at the Canadian border, the western extent is at the edge of the image (127° west) and the south and east extents are the same as in the previous map.
