Tips for Stitching Together Images for a Base Map

Existing maps, aerial photography, and screenshots are good starting points for building a custom map. But maybe that large map doesn’t fit on your scanner. Or your area of interest is more extensive than shown on that web map’s window. As a result, you have multiple images that need stitched together in Ortelius. The following tips will help you achieve the best results.

Import images using the Image Browser or drag-dropped directly from Apple’s Finder. Images in Apple’s Photos/iPhoto app may be imported through the Ortelius File menu. Screenshots copied to clipboard may be pasted directly onto your map canvas. Use a separate layer to contain each complete base map.

Capture Images at the Same Zoom Scale and Resolution

It is important to preserve a consistent scale among your tiled images. When scanning multiple sections of a paper map, be consistent with the resolution at which the document is scanned. When capturing screenshots from a web map, keep the web map at a consistent zoom scale as you pan around for screen shots (don’t zoom in and out on the map).

HINT: Overlapping recognizable features can be used to help register images from different sources, time periods, or captured at different scales. (see below)

Capture Overlap

When possible, include overlapping recognizable features with each scan or screenshot. Overlapping features are used as registration points when aligning the images. For example, include a major road at the bottom of one image (1) and again at the top of the next (2). When stitching images together you will overlap the features for perfect alignment. When your map is larger than your scan bed, fold the map (or reposition on the scanner) in such a way as to capture the overlap.

Include the Scale Bar

If your source map has a scale bar, include it with your scans or screenshots. After you are finished aligning and resizing your base map, you can use the Ortelius Ruler tool to calibrate the scale of your new map.

Be Consistent When Resizing Images

Resize images consistently to preserve a consistent scale among them. Because individual scans and screenshots may have slightly different dimensions, it is best to resize images by a percent factor. Either resize image all at once, or resize all by the same factor (i.e., 50%). Here’s how:

Manually resize – Select all your images. Use command + G to group them. Hold the shift key to maintain the aspect ratio, then manually drag the group’s object handles to resize.

Geometry Tab – Select all your images. In the Geometry Tab, check the box to use % for units. In the width and height setting, click the lock to lock aspect ratio. Enter a percent value to resize objects.

HINT: You may need to use images from different sources, time periods, or captured at different scales. This may require resizing different source maps separately to align recognizable features. Be aware that maps from different sources may also use different projections, affecting their scale properties and ability to neatly align with other maps of the same area. Capture the (approximate) scale of the predominant map.

Use Image Opacity While Aligning Images

Use image opacity to view overlapping areas of images. Select one or more images and adjust the image opacity in the Geometry Tab. Overlapping features, such as road intersections, buildings, and water, will be used as registration points. Once overlapping features are visible in both images, drag to move the images as needed to align them. When finished, change the opacity setting back to 100%.

HINT: After aligning images, group them to make final adjustments to the overall size and position of the base map on your drawing canvas.

Crop Images As Needed

If your overlapping images contain edge elements you want to hide, individual images can be cropped. Double-click an image to activate the cropping handles. The bounding box will change to a dashed yellow line. Move bounding box handles to crop.

Layer Opacity Hides Seam Lines

You may want to show a semi-transparent image as you trace features, or for display behind your final map. To adjust base map opacity without seeing the overlapping areas (seam lines) between images, use Layer Opacity. Begin with individual images set to 100% opacity (set image opacity in the Geometry Tab).

Open the Layers Tab. To expose the layer opacity slider, right-click (two fingers on the track pad) the layer ‘eye’ icon of the layer containing the images. Slide to adjust layer opacity. Note, the ‘eye’ icon changes to indicate opacity has been applied.

Keep File Sizes Small

For best performance, limit image file sizes while still meeting your project requirements. Some considerations include 1) the size of your final map, 2) whether or not your image base map will be visible in your final map, and 3) your presentation format resolution requirements (e.g., print, on-screen). For example, if your map will be printed and includes a visible aerial photography background image, then 300dpi image resolution may be appropriate. If it is to be viewed only on-screen, 96dpi may work best. On any given project, determining your requirements from the beginning is a best practice – particularly if you have publication standards that must be met.

Here are some ways to reduce image file sizes before importing into Ortelius:

  • Resolution – If your base map is very high resolution (e.g., 600dpi), a lower resolution may be adequate to trace the features of interest. Use Apple’s Preview app (or other software) ‘Tools > Adjust Size…’ to lower the image resolution.
  • Dimension – If you are starting with a large base map, but your end product is a smaller map, reduced dimension may be adequate for you to trace the features of interest. Use Apple’s Preview app (or other software) ‘Tools > Adjust Size…’ to reduce the size. If you are changing the size of multiple image tiles, make sure to be consistent by using percentages, e.g., reduce by 50%.
  • Format – File type influences file size, while it may not impact your requirements. For example, a JPG image may be smaller than its PNG equivalent, which may both be smaller than PDF. It may take some experimentation to determine which works best for your requirements. Use Apple’s Preview app (or other software) to export or save as different formats.
  • Compression – Compression software can significantly reduce file sizes with no perceptible impact on image quality. Our current go-to for JPG and PNG compression is the online service TinyPNG.

HINT: Keep copies your original high-resolution files, and use these methods to reduce the size of duplicates.

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