Map-based questionnaires often benefit from a base map with an image layer on top to provide additional context, such as a site plan or floor layout. There are two primary ways to add these image layers to your maps: georeferenced images and Cloud Optimized GeoTIFFs (COGs).
Georeferenced images
Georeferencing an image is a straightforward way to overlay a small image file onto a map. The process involves manually positioning and scaling an image so that it aligns with a specific geographic location. This method is most effective for small images.
When to Use:
- Small Areas: Best for small-scale layouts like a single property, a floor plan, or a small field..
- Simple Needs: Ideal for situations where an approximate, manually-aligned overlay is sufficient.
- Quick & Easy: You can upload these images directly to your team and manually adjust their placement.
Important Considerations:
- File Size: Make sure that the image is a reasonable size as heavier images may be difficult to load on some respondents' devices. It is difficult to give a specific size recommendation as the optimal size will depend on multiple conditions (e.g. how many images does the same survey page have in total). Make sure to test your questionnaire on different devices to check performance.
- Manual Adjustment: You'll need to manually resize, rotate, and place the image on the map, which can be less precise than using a GeoTIFF.
How to Georeference:
Upload the image to Materials by clicking New.
Click Georeference.
Zoom into the correct area on the map. Click roughly where you want to place the image. Adjust the size by dragging the corners of the image, and rotate it by dragging the circle in the bottom right corner.
You can check that the placement is correct from the top right corner menu.
- Click Save to finish.
Go to the questionnaire. Set "Map" as the page type and select a basemap. Add the image by selecting it from the drop-down menu under Map overlay.
GeoTIFFs
Cloud Optimized GeoTIFFs (COGs) are a specialized format of GeoTIFF designed for efficient web-based use. They are regular GeoTIFF files aimed at being hosted on a HTTP file server, with an internal organization that enables more efficient loading on the cloud. The conversion process restructures the file, adding an internal overview and tile organization that optimizes it for web use. Maptionnaire can then request only the specific "tiles" of the image that are visible on the screen, rather than downloading the entire file at once.
When to Use:
- Larger Images: COGs are ideal for larger-scale images, such as high-resolution aerial photographs, large site plans, or city-level maps.
- Detailed Maps: The format preserves detail, making it suitable for maps with fine lines, text, and other intricate features.
- Efficiency: Because they load incrementally, COGs prevent the questionnaire from crashing and provide a smooth user experience.
Important Considerations:
- Conversion: You can create a GeoTIFF using GIS software like ArcGIS or QGIS. You can then send the GeoTIFF to our team for conversion to COG.
- File Size & Quality: The original GeoTIFF file should typically be less than 25 MB, although we can compress larger files. Extreme compression to reduce size may result in a loss of fine detail, particularly text.
If you'd like to use a GeoTIFF, please contact support@maptionnaire.com and we'll guide you further!