Georeferenced Layer on the Map
Georeferenced Layer on the Map

Follow these three steps to create a map layer from a plain image.  Each step is explained in detail below (click on the step item to skip to its details):

Copy the image file to where Bit Map can access it

Georeferencing is the process of configuring how each pixel in the image relates to coordinates in a particular spatial reference system and vice versa;  without georeferencing, there would be no way for Bit Map to know where in the world a map image fits, or where in the map image your current location or other features fit

This step can be skipped if the image is already georeferenced – eg, GeoTIFF

This step is automatically triggered by the next step if the image is not already georeferenced;  therefore it is not usually necessary to explicitly invoke this step unless you wish to change the georeferencing of an image that is already georeferenced

Images are split into a grid of tiles (in fact into several grids of tiles for different zoom levels) to avoid the memory-intensive and slow process of attempting to load an entire image into memory when displaying it as a map layer.


Import Image to Bit Map

The image will need to be imported into Bit Map’s own ‘Documents’ repository where it has full access to the image file.  Follow thee steps below to import the image.

NB:  The ‘Files’ app (from Apple) will be required.  If you do not already have this app, or if you’ve deleted it, you will need to download the ‘Files’ app from the App Store.

  • Copy (or move) the image into the ‘Files’ app on your device.
    • For most users, the easiest way to do this would be to copy the image file to the ‘iCloud’ folder on your device (or on your computer after which it should sync to your device).
  • In the ‘Files’ app, tap on the image file
  • Tap on the image file’s share/action button (bottom left)
  • Tap on Bit Map in the list of apps that appears in the share sheet (second row, you may need to scroll the row across to see it)
    • Bit Map will open and ask for confirmation to import the file to your Bit Map library 
    • If Bit Map does not appear in the list, it may be an unsupported image type
  • Tap ‘Import’ to import the file to Bit Map’s library

     


Georeference Image

NB:  This step is usually not required to be invoked explicitly, as the ‘Create Tile Grid Layer’ step will automatically trigger this step if the image is not already georeferenced.

Therefore you can usually skip explicitly invoking this step and jump straight to the Create Tile Grid Layer step (which will automatically include this step, if necessary).

However, you can invoke georeferencing explicitly if you wish to georeference the image without creating a map layer or if you wish to replace the georeferencing configuration.  To commence georeferencing without creating a layer:

  • From the Main Map View, tap the Layers Library button to display the Layers Library
  • In the Layers Library, tap the image file name
    • Bit Map will display an Image View including:
      • Image
      • Type of georeferencing used for the image (if any)
      • Spatial reference system used for the image (if any)
  • Tap ‘Georeference’, at the bottom-left of the Image View to commence georeferencing the image

To continue georeferencing, either after explicitly invoking this step (as above), or as part of creating a tile grid layer (as below):

  • Select ONE of the following four methods for georeferencing (in order from simpler/less-accurate, to more-complex/more-accurate):
    • Match Map
      • Simply pan zoom the map (including all the layers from the main map view) until its features line up with the features of the image (the image will be georeferenced for the spatial reference system currently being used by the map, even if this is not the original spatial reference system of the image)
    • Extent (min/max) Coordinates
      • Enter the coordinates of the bottom-left and top-right corners of the image (and specify which spatial reference system the coordinates are for)
    • Control Points by Coordinates
      • Enter the coordinates of at least two arbitrary control points on the image (and specify which spatial reference system the coordinates are for)
    • Control Points by Map Points
      • Place at least two arbitrary control points on the image and corresponding control points on the map (the image will be georeferenced for the spatial reference system currently being used by the map, even if this is not the original spatial reference system of the image)
  • Refer to the relevant section below to continue with on with the selected georeferencing method

Georeference Option:  Match Map

This is the easiest georeferencing method.  However, it can be less accurate, if the image was generated using a different spatial reference system to the current map or if the image is warped (eg from folds in paper).

The resulting layer will be assigned the same spatial reference system as the current map, even if this is not the spatial reference system that was used to generate the image.

At any time during this process:

  • Drag the slider to change how transparent/opaque the image and map appear as they overlay each other

Georeference as follows:

  • Use two fingers to drag and/or pan the map until its features match the corresponding features on the image as closely as possible over as much of the image area as possible.
    • Note that if the image is not in the same spatial reference system as the map, or is warped by folding (paper), etc, then an exact match may not be possible
  • Tap ‘Done’ at top-right

Georeference Option:  Extent Coordinates

This georeferencing method is easy if the coordinates of the map extent (corners) are known.  However, it can be less accurate, if the image was generated using a different spatial reference system to the current map or if the image is warped (eg from folds in paper).

  • From the list of Spatial Reference Systems that appears, select the one used by the image (or the one for which you know the maximum/minimum coordinates of the image)
    • Use the search field to filter the list of SRSs
  • Enter the four coordinates:
    • Minimum X (left)
    • Minimum Y (bottom)
    • Maximum X (right)
    • Maximum Y (top)
  • Tap ‘OK’

Georeference Option:  Control Points by Coordinates

This method of georeferencing requires a little more effort, but should result in a more accurate result.  It requires knowledge of the coordinates of a few features on the map (map grid lines can be useful for this, if they are present).

At any time during this process:

  • Use TWO fingers to pan/zoom the image
  • Use ONE finger to move the target

Georeference as follows:

  • From the list of Spatial Reference Systems that appears, select the one used by the image (or the one for which you know the maximum/minimum coordinates of the image)
    • Use the search field to filter the list of SRSs
  • Place the target (using ONE finger) at a point on the image where the coordinates are known
    • Zoom in (and pan) the image to be more accurate (TWO fingers)
    • Intersections of grid lines are excellent candidates for this, if the image includes them
  • Tap the ‘+’ item (bottom) to place a control point at the target location
  • Enter the coordinates for this control point
  • Tap ‘OK’
  • Repeat for more control points
  • Tap ‘Done’

NB:  At least TWO control points are required.  At least THREE are recommended for accurate georeferencing.  The further apart the control points are in both X and Y axes, the more accurate the georeferencing will be.

Georeference Option:  Control Points by Map Points

This georeferencing method takes some work to configure, but should result in a more accurate result.  It requires being able to match a few features on the image with their corresponding features on the map.

The resulting layer will be assigned the same spatial reference system as the current map, even if this is not the spatial reference system that was used to generate the image.

At any time during this process:

  • Use TWO fingers to pan/zoom the image OR the map (depending on which is selected)
  • Use ONE finger to move the target
  • Select the ‘Move Map’ or ‘Move Image’ item (bottom) to change which of the map/image is panned/zoomed when using two fingers
  • Drag the slider to change how transparent/opaque the image and map appear as they overlay each other

Georeference as follows:

  • Place the target (using ONE finger) at a point on the IMAGE where the corresponding map point can be easily found
    • Zoom in (and pan) the image to be more accurate (TWO fingers, ‘Move Image’ selected at bottom)
  • Tap the ‘+’ item (bottom) to place a control point at the target location on the image
    • The ‘Move Map’ item will automatically become selected
    • The ‘+’ item will become highlighted to indicate that you now need to define the corresponding point on the map
  • Place the target (using ONE finger) at the point on the MAP that corresponds to the point that has just been placed on the image
    • Zoom in (and pan) the map to be more accurate (TWO fingers, ‘Move Map’ selected at bottom)
  • Tap the ‘+’ item (bottom) to place a control point at the target location on the map
    • The ‘Move Image’ item will automatically become selected
    • The ‘+’ item will return to its normal state to indicate that you can now place another point on the image
  • Repeat for more control points
  • Tap ‘Done’

NB:  At least TWO control points are required.  At least THREE are recommended for accurate georeferencing.  The further apart the control points are in both X and Y axes, the more accurate the georeferencing will be.


Create Tile Grid Layer From Georeferenced Image

  • If not already showing the image in Bit Map’s Image View, navigate to the Image View:
    • From the Main Map View, tap the Layers Library button to display the Layers Library
    • In the Layers Library, tap the image file name
      • Bit Map will display an Image View including:
        • Image
        • Type of georeferencing used for the image (if any)
        • Spatial reference system used for the image (if any)
  • Tap ‘Create Layer’ at the bottom-right of the Image View to commence creating a Tile Grid Layer from the image
    • If the image is not already georeferenced, then the georeferencing process will begin
      • Refer to the ‘Georeference Image’ section, above, to georeference the image
    • The image will be automatically be reprojected (warped) if necessary
    • The image will be split into a grid of tiles for the maximum zoom level (ie, it’s native size) and then again for each zoom level lower until only one tile is required to contain the entire (zoomed out) image

The layer will now be available in the Layers Library as a separate item to the original image file.  If an intermediary file was required (eg, if the image had to be reprojected before being tiled), then a third ‘.tiff’ (GeoTIFF) item will also be available.  This intermediary and the original can now be deleted to save space (and to make the library easier to browse) if you wish.

To add the layer to the map:

  • Tap the ‘Library’ (back) button to return to the Layers Library view
  • From the Layers Library view, tap on the Add to Map button next to the Tile Grid layer item to add that layer to the map
    • The layer will be added to the top of the layer stack, and may obscure layers beneath it;  layer drawing order can be modified in the Map Layers view