The New Martenet Atlas

The New Martenet Atlas is the primary access portal to our online digital archive. Through it you may determine whether we have survey records relating to a particular area, and get the information necessary to give us in your request for access.

You will notice that, except for Baltimore City, the entire state of Maryland is superimposed with a grid. Each square of the grid is 3,000 feet by 3,000 feet, and, except for Montgomery and Prince George's counties, corresponds to the state tax maps and grids. (Montgomery and Prince George's counties now have a different tax mapping scheme, but, for a number of reasons, we elected to maintain the original state-wide scheme across those counties as well.) Baltimore City uses a "block" system that we have reproduced here, except our blocks occasionally do not coincide with Baltimore City's blocks, and, in other cases, our numbering is different. Always use the SJM block numbers when requesting access.

Clicking on any location on the map will pop up a window listing the county, map and grid (or block) of the location and listing the number of records we have pertaining to that area. Nearly all of our records are geographically coded as such, and this method provides the easiest way to identify records of interest. In Baltimore City, the blocks do not encompass the adjoining streets, but if you click within 20 feet of a street line, the popup window should appear.

Operating Controls

You can pan to your area of interest by simply dragging the map (using left mouse button) over to the desired location. Zoom in or out using the mouse wheel or by using the navigational bar near the upper left corner of the map. Alternatively, you can zoom into a selected area by holding the shift button down and windowing the desired location.

No Popup Windows?

Make sure you have set your browser to allow popup windows from And you must use the address, instead of merely Omitting the leading www's results in no popup windows. Also, as noted above, in Baltimore City the blocks do not include the adjacent streets. Clicking farther than 20 feet from a street line will not popup a window.

"What's Near Me?"

In the bottom center of the screen is a button labeled, "What's Near Me?" This function uses the geo-location functionality of modern browsers to determine (1) where you are, and (2) what records we have near your location. After clicking the button, your browser should ask for permission to use geo-locational data. Select yes and a positional reliabilty box will pop up. Select OK and a new window will popup listing either the number of records we have in your specific area (if you're not logged in) or the records themselves (if you are logged in).

There is a caveat, however. One of the numbers generated by the browser is the reliability of the position. Expressed as a radius, it's basically an error circle. Your position should fall within that circle, the center of which is the location it returned, and the radius is the uncertainty. If that uncertainty is greater than 1,500 feet, the process aborts. We think that if that much unreliability is embedded in the returned position, it's not good enough for record retrieval. Our experience has been that desktop machines and other non-mobile devices typically report very large unreliability numbers, and are thus unsuitable for that task.


In the lower right of the screen your system displays real-time coordinates in five different coordinate systems as you move your mouse around the map. Clicking the checkbox next to any (or all) of the systems will highlight that system's coordinates. Note that because of the anomolies inherent in the NAD27 system, the coordinate readout for NAD27 here is only within 10 feet or so. (The SJM grid lines throughout the map are on even 3,000-foot NAD27 grids and use a much more rigorous algorithm for placement. You can compare the readout in your area of interest with that at the nearest grid line to approximate the readout error. Generally, it is within 10 feet all over the state.) The Washington Suburban Sanitary Commission (WSSC) coordinates use an old conversion formula which, we are given to understand, was later superceded by a more accurate solution, but in the time since that agency moved to the NAD83 system itself, the later conversion was lost. Those values are also dependent on the NAD27 values, and so have two sources of uncertainty. We estimate those readouts should be within 20 feet or so.

Changing the base layer

Our index grids (the light grey grids) overlay one of several base maps. The default is Google Map, but other options include OpenStreetMap, Google Hybrid and Google Satellite. (Hybrid is the satellite image with street names and locations labeled.) You may change the base map by clicking on the little plus near the upper right corner of the map. A menu will pop out listing the available base layers and the SJM Index layer. Select the desired base layer by clicking the radio button next to it. The map image will automatically update while maintaining your location of interest. You can turn off the SJM Index by unchecking the button next to the SJM Index entry. The SJM Index must be on for the pop up windows to function, however. (No grid--no results, in other words.)

If you are logged-in to our site, clicking on the area of interest will display the actual list of records pertaining to the location, not simply the number of hits. Your access privileges determine whether or not the records themselves display.

As always, we're looking for feedback. If you discover something that doesn't seem to work as advertised or is clumsy, please let us know.