The Save PhotoMaps option can be opened by Clicking the camera tool icon within the MapBrowser toolbar. User have two options when saving a PhotoMap: Current Screen and Define Area.
The Current Screen setting will download the whole window at exactly the resolution you see. There's no choice of projection with this setting, image downloads will be in EPSG:3857, Spherical Mercator (see Projection below).
If you choose Define Area, a suite of options will appear and a bounding box will show on the map.
Projection is an important setting. When the spherical Earth is "projected" onto a flat map surface, distortions occur. Like Google, Bing, and MapQuest maps, we use Spherical Mercator (EPSG:3857, aka EPSG:3785 and 'EPSG':900913) in our online MapBrowser. This maps the sphere to a convenient rectangle, but has large distortions away from the Equator.
When saving images, we recommend using a different map projection (such as these for Australia) so that scale and measurements are accurate.
Resolution allows you to select the resolution that you would like in meters per pixel. The available options are determined by the bounding box you select. Please note that if saving in Spherical Mercator, meters/pixel is only a rough estimate due to that projection's inherent distortions.
There is a system limitation to maximum image size, which varies by coordinate system. For the default Web Mercator coordinate system, this limit is 50 megapixels. Due to the way local map projections are rendered, the maximum image size for locally projected imagery may be lower than 50 megapixels. If your bounding box is too large for the highest resolution you will see the message "For a higher resolution please define a smaller area." This tool permits saving of images at extremely high aspect ratios, which can be very useful when saving along transportation routes.
Width and Height indicate the dimensions of the area you've selected to download, based on the chosen resolution.
The Include georeference file checkbox determines whether or not a JGW sister file will download alongside your JPG image in a ZIP file. A JGW is a simple text file containing six numbers for locating the image in a coordinate system:
- Pixel size in the x-direction in map units
- Rotation about y-axis (which is zero for this system)
- Rotation about x-axis (which is zero for this system)
- Pixel size in the y-direction in map units (often negative)
- X-coordinate of the center of the upper left pixel
- Y-coordinate of the center of the upper left pixel
The JGW georeference file has the same filename as the image file, and its name must always match that of the JPG. The filenames follow this format:
For example, an image downloaded in the EPSG:28355 (MGA 55) projection from a July 9, 2014 survey centered at (-21.060746,149.209422) with resolution of 1.194 meters per pixel would have the filename:
You can import the image and its JGW to any application that supports georeference files. Examples are AutoCAD Map 3D or Civil 3D, ArcGIS, ArcMap, MapInfo and QGIS. Our GIS and CAD Integration articles describe how to do this for popular packages.
File Size is a very rough estimate as to the maximum file size that will be downloaded. The actual size will often be much lower because it is difficult for us to estimate image compression efficiency in advance.