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Add historic or other georectified maps to ArcGIS online as an image layer. Written and Edited by Austin Mason, Reviewed by Aaron Young |
We are going to add a tiled image layer to an ArcGIS online map. One common reason for doing so would be to add a historic map overlay to your project, or as a first step before digitizing details on a physical map to create new point, line or area features.
You will need:
- An ArcGIS Online account
- A georectified map hosted publicly online offering a tiled service
ArcGIS Online is a widely used professional web mapping application. Many industries and higher ed institutions maintain subscriptions, so check with yours to see if you have access to an account.
{% hint style="info" %} If you don't have access to ArcGIS Online, you can sign up for a free public account that offers basic map making capabilities. {% endhint %}
A georectified map is an image that has geographic coordinates embedded in its metadata. With GIS software you can make your own, but they can also be hosted online and shared in various ways. We will use an already rectified map.
{% hint style="info" %} Some potential sources of rectified map images are:
If there's not an already georectified map online, you need to follow the following steps.
- Find a map! While the websites listed above are excellent places to look for rectified and non-rectified maps, (MapWarper.net, New York Public Library MapWarper, David Rumsey Map Collection) you can also find map images through archives or other databases.
- Mapwarper accept most image formats. When you upload maps onto mapwarper, they will usually convert the map into a png file on its server. Hence when you copy the URL over to ArcGIS, they will have the ".png" extension in the url.
We are going to add this map as web tiles so it will be served at higher resolution as we zoom in, the same way satellite map layers get displayed on Google and other maps services.
1 - Find a georectified map. You can use the one below from MapWarper.net for this demo:
Davison's Pocket Map of Minneapolis, 1886
2 - Get the URL in Tiles or XYZ format, which should follow the format below
https://mapwarper.net/maps/tile/38325/{z}/{x}/{y}.png
The
3 - In ArcGIS Online, open a new or existing Map
4 - From the Add Menu, choose Add Layer from Web
5 - In the data type drop down of the window that opens, choose Tile Layer
5a - Paste and edit the Tile URL to match the ArcGIS format
{% hint style="warning" %} This is the crucial step — change {z}/{x}/{y} in the Tile URL to {level}/{col}/{row} as stated in the ArcGIS help text and illustrated below. {% endhint %}
5b - OPTIONAL - Set the Extent to match the content you are mapping, e.g. in this case Minneapolis, MN.
6 - Click Add Layer and see your map overlaid.
{% hint style="info" %} The "Title" you type in will be the name of the tile layer in ArcGIS.
The "Credits" you type in can help you keep track who created which tile layers. {% endhint %}
The biggest challenge with displaying an image on a zoomable map is resolution. Digital images are raster data with a fixed number of discrete cells or pixels giving the image its resolution. High resolution images are large files, which can take a long time to load over the internet.
{% hint style="info" %} Read about the difference between raster and vector data types for spatial data here. {% endhint %}
Tiled web maps (sometimes called slippy maps), pioneered by Google Maps, get around this problem by breaking up a large image into a series of square tiles of different resolutions for different zoom levels. Only the tiles within the view frame at the current zoom level are loaded and swapped out for different ones when the user pans or zooms.
The basic principles are the same in all web mapping tools, but the format can vary. The key trick for using tiled web resources in ArcGIS online is to alter the URL to accommodate their format.
{% hint style="warning" %} The crucial step to make tile maps work for ArcGIS Online is to change {z}/{x}/{y} in the Tile URL to {level}/{col}/{row} {% endhint %}