GIS Help

Learn how to make your own Gmap4 maps that display
ArcGIS MapServer and ImageServer data
including attribute values
List of 50+ federal GIS servers
List of 150+ state GIS servers


Gmap4 can display data that is hosted on GIS servers. You do not need to be a GIS expert to do this! All you need to do is follow a few simple instructions. This page will show you what you need to know in order to be able to (1) identify data on GIS servers that Gmap4 can display and (2) make a Gmap4 link to display that data. Users of your map will be able to click a GIS symbol on your map and see a popup with all the attribute data the GIS server has for the thing that was clicked.

Part 1 - About GIS server directories

Gmap4 can display GIS data that is available via REST services. These services are arranged in a 'top-down' manner just like the folders and files on a harddrive. The GIS server can display a series of web pages that lets you navigate through the directories on the server and see lists of the data layers that are available.

The following link displays a page from a GIS server operated by the U.S. Forest Service.

Each time you are looking at a Gmap4 map and click on a GIS symbol you then see a popup with data ("attributes") about the thing you clicked. Near the top of that popup there is a link titled "GIS server directory". If you click that link then you will see a web page similar to the above example from the Forest Service. This documentation will help you understand what those web pages are showing you.

At the top of each GIS directory page it says "ArcGIS REST Services Directory". Underneath that title is a line that has several links to different places in the directory tree. Here is an example of such a line:

     Home > services > EDW > EDW_RecreationOpportunities_01 (MapServer)

Click on "Home" or "services" and you go to the top of the REST services. You can then start clicking links to drill down and explore the various data layers that are available on the server.

NOTE: On this Forest Service server (see the above link) their "authoritative" data is under the EDW directory. All of the other directories should be considered temporary. The data in those other directories can disappear at any time.

The most common type of data that is available via REST services is called MapServer. Gmap4 can display all MapServer data. Gmap4 can also display all ImageServer data. You might also see data that is identified as FeatureServer or GPServer as well as several other types. Gmap4 cannot currently display these other data types.

As you navigate around a GIS services directory it is easy to recognize layers that Gmap4 can display. Just watch for web pages with a heading at the top that includes "(MapServer)" or "(ImageServer)". Here is an example of such a heading:

     EDW/EDW_RecreationOpportunities_01 (MapServer)

A web page with "(MapServer)" in the heading will include a section that is titled "Layers". That section lists one or more layers of data that Gmap4 can display. Each layer has a name and a number. The numbers always start at '0' and are important since they are used to identify the data layers.

Sometimes when you are looking at a list of MapServer layers you will see useful metadata under the "description" heading or other headings.

If you click on a layer name and scroll to the bottom of the page that appears, then you will see a list of attribute names. These same attribute names (along with any data for each one) will appear in a popup when you are looking at a Gmap4 map and click on a GIS symbol for this layer.

As you look at GIS service directories you will notice that often the attribute names are so cryptic as to be meaningless by themselves. Another problem is that attribute values are sometimes coded. These problems could be solved if one of the attributes for the data layer was a link that pointed to a data dictionary that provided a short narrative description of the data contained in each attribute field and an explanation of any codes that are used.

Now look again at this Forest Service page:

Click the link that says "Legend" and you will see all the different symbols this layer can put on the map and a description of each one. Usually a Gmap4 map that displays GIS data should have some way to help the user find the corresponding legend on the GIS server.

Part 2 - About using Gmap4 to display GIS data

You only need to know four things in order to display GIS data with Gmap4. And one of those things you literally make up. Below are several links that walk you through the process of displaying GIS data with Gmap4.

Here is a Gmap4 link that does not display any GIS data.,-121.479778&z=15&t=t4

The above map opens zoomed in on a spot in the Mt. Baker-Snoqualmie National Forest east of Seattle. The ll parameter gives the center of the map, the z parameter specifies the zoom level and the t parameter specifies the basemap to be displayed which in this case will be a high resolution USGS topographic map.

Displaying MapServer GIS data with Gmap4

Now we are going to add a "rest" parameter to the above map link. The rest parameter specifies the GIS MapServer data that the map can display. Each MapServer rest parameter is made up of 4 parts:
All 4 parts must be present.

Here is the rest parameter we will add to the map link:

The value of the rest parameter is the internet link that points to the GIS directory that has the data layer you want to display. You get this address by using your browser to navigate around the services directory of a GIS server and when you find a layer you want to display then you simply copy the GIS directory address from the address bar of your web browser.

If you are curious, here is a link that will open the GIS services directory described above.
Notice that this directory only has a single layer which is number 0.

The value of the name parameter is a short name that you make up. Do not use any spaces in this short name. Use an underline character instead of the space character. This name is used by Gmap4 in the dropdown menu that shows the GIS overlay layers that can be displayed by that Gmap4 map. If the name is too long then it will not fit on one line when you display the Gmap4 dropdown menu that shows the GIS overlay layers your map can display.

The layers parameter is the number of the GIS layer as shown by the services directory of this GIS server. This can be a single number, a comma separated list, a range of numbers (layers=3-7) or a combination (layers=0,3-7,12). Usually you will likely only use a single number as the value for the layers parameter.

If the transparent parameter is set to true, then Gmap4 will treat this layer as an overlay. If this parameter is set to false, then Gmap4 will treat this layer as a basemap. Most MapServer data should be added to the map as overlays. Doing it this way allows that MapServer data to display on top of any of the basemaps.

Below is the same Gmap4 link that was shown near the start of this section. The link now includes the parameters that specify the GIS layer that shows recreation opportunities in the national forests. When you open this map link, the GIS overlay layer is 'off'. To turn this layer 'on' click the button in the corner of the map that always shows the name of the current basemap (Basemap button). Unless you have changed the basemap it will say "t4 Topo High". Then in the Overlay dropdown menu, click "Recreation_site" to turn on this GIS overlay layer. Mobile users: touch the basemap button and then scroll down to the overlay section.,-121.479778&z=15&t=t4&rest=

If you want the "Recreation_site" overlay layer to be 'on' when the map opens, then you need to tweak the t parameter like so: &t=t4,Recreation_site. The t parameter can be used to specify the basemap to appear when the map opens and one or more GIS layers that will be 'on'. Use the short names you made up for the GIS layers and separate the names with commas. Do not let any spaces creep in to the Gmap4 link.

Here is the finished Gmap4 link that will open with this GIS overlay layer turned 'on'. You can click the GIS symbol that is displayed and see a popup with all the attribute data the GIS server has for the thing you clicked.,-121.479778&z=15&t=t4,Recreation_site&rest=

Displaying ImageServer GIS data with Gmap4

Most ImageServer data is aerial photos. The syntax for displaying ImageServer data is a bit easier since by default Gmap4 treats ImageServer data as an additional basemap instead of an overlay.
Each ImageServer rest parameter is made up of 2 parts:


The "rest" parameter is the complete https or http link that points to the data on the GIS server.
The "name" parameter is a short name that you make up. Use an underline symbol instead of a space.

Below is an example map that displays aerial photos for the state of Ohio. This ImageServer data is hosted on a GIS server operated by the state. When the map opens it is zoomed in on a random spot and the basemap "Ohio_aerial" is displayed.

Ohio aerial photos:,-82.267920&z=17&t=Ohio_aerial&rest=

*** Note on the above map the trail running north-south. Now switch to the Google aerial and the trail disappears since the aerial photos Google usually uses are taken when the leaves are on the trees. By contrast, states and local governments may have aerial photos taken when there are no leaves on the trees and as a result you can see the ground.

If you look carefully at the above link you will see that the index page on the GIS server for this ImageServer data is:

If you visit this GIS index page then you could click the "Home" link near the upper left corner of your screen and go to the top of the index for this GIS server. You could then start drilling down on the "folders" and "services" to see if this GIS server has any other data you would like to look at with Gmap4.

Now you know how to display most GIS MapServer and ImageServer data with Gmap4. However, to avoid frustration there are two special kinds of MapServer layers that you should know about.

Part 3 - Special GIS layers and some 'tips'

The first type of special MapServer layers can be see on this GIS directory page from the state of Washington:

Notice that the name "Water Access Sites" is used for layers 1, 11, 21, 31, 41, 51. This means that each individual layer number will only display data at certain zoom levels. If you want your map to display "Water Access Sites" at the most zoom levels possible, then you need to write your layers parameter like so: &layers=1,11,21,31,41,51

An example of the second type of special MapServer layers is seen here:

At first glance this looks like a normal list of GIS MapServer layers. But scroll down below the list of layers and notice where it says "Single Fused Map Cache: true". This means that all the layers listed on this page have been combined (i.e. fused) into a single thing. Usually this will be intended to be a basemap and you would write the rest parameter like so:


Remember, in order to tell Gmap4 to treat this data as a basemap, set the transparent parameter to false. Also, usually for a "Fused Map Cache" you set the layers parameter to a range that includes all the layers. If that does not work, then set the layers parameter to just 0.

In addition to specifying GIS data by including parameters (rest, name, layers and transparent) in a Gmap4 link, you can also include those same parameters in a text file. This approach is useful when you want to make a Gmap4 map with multiple GIS data layers that can be turned on/off. You can see examples of Gmap4 maps that use text files by going to the MappingSupport project page and scrolling down to the section titled "Gmap4 Special Maps". The internet address for each text file is part of each Gmap4 link. You can download any of those text files and take a look at them. Also, each of those maps has a "Map tips" link in the upper left corner with the map legend and more information.

You can put your text file online at Google Sites for free. The Gmap4 Help page has a pdf file titled "Working With Files" that contains step-by-step instructions for working with Google Sites.

Tip: If you want to share a Gmap4 map that displays GIS data with someone then first make the map look the way you want it to look on your screen and then click Menu ==> "link to this map". The link that is displayed will replicate the map on your screen.

Tip: You can use Google to search for GIS servers. After all, the directory pages that GIS servers display are web pages and Google indexes web pages. Here are two good Google searches for finding GIS servers with REST services:

     ____________ "ArcGIS REST Services Directory"
     ____________ "rest/services"

Replace the underline with one or more search terms. In addition to the federal government and states (see links at the top of this page), many counties and cities have GIS servers with public data. For example, if you search on:

     San Bernardino County "rest/services"

then you will easily find one or two GIS servers for this county.