The Fiske Center Blog

Weblog for the Fiske Center for Archaeological Research at the University of Massachusetts Boston.

May 11, 2022
by zachary.guttman001

Photogrammetry at the Jeremiah Lee Mansion

In April of 2022 the Fiske Center for Archaeological Research began an exciting project in Marblehead Massachusetts.  The Marblehead Museum is interested in understanding the yard space around the Jeremiah Lee Mansion, a Georgian mansion built in 1768 for Jeremiah Lee, the wealthiest merchant in colonial Massachusetts at the time. 

More specifically, the project seeks to investigate the space on the east side of the house, between the Mansion and the Brick Kitchen.  This brick building dates to the same time period and may have housed the mansion’s enslaved residents or domestic servants.  By investigating this lot, we are hoping to encounter older 18th century archaeological deposits, deposits from the Lee period, or deposits and foundations from later buildings that occupied this space.

In preparation for a ground penetrating radar survey in late April and excavations in June, the first step was to georeference all the old maps, which was described in a previous post. The next step is to create an orthophoto.  This is an aerial image that has been geometrically corrected (ortho-rectified) so that the image has uniform distance measurements across the entire photo and is perfectly top down.  Unrectified aerial or satellite images suffer from terrain effects and distortion from the camera lens and the angle that the photo was taken, which can cause problems in later processing steps.  You can see these kinds of images on google earth- notice the sides of the buildings.

To create the orthophoto, ground control points were laid out across the site and a large number of drone photographs were taken. 

Paper plates
Drone photo showing different color paper plates used as ground control points

Ground control points are places on the ground (in this case paper plates) with known coordinates associated with them.  These points are used to tie map coordinates from the aerial photos to precise points on the earth, thereby yielding an accurate product. John Schoenfelder flew the drone and did much of the mapping. 

Lee Mansion drone
John Schoenfelder flying drone in front of Lee mansion

Using the photogrammetry software Agisoft Metashape, we created an orthophoto of the Lee Mansion grounds.  This software requires you to build a “sparse cloud” and a “dense cloud”, which are used to generate the orthophoto.

Lee mansion orthophoto
Orthophoto of Jeremiah Lee Mansion. The photo is from a perfectly top-down orientation.
Lee mansion sparse cloud
The sparse cloud generated in Agisoft Metashape
Lee mansion dense cloud
Dense cloud generated in Agisoft Metashape

These are both types of point clouds that exist in three-dimensional space.  The sparse cloud represents the tie points for overlapping image pairs while the dense cloud generates depth maps for these overlapping pairs and merges them together to create a three-dimensional model of the site.  The dense cloud was used to create a digital elevation model of the site, which was then used to create the final orthophoto. 

Lee mansion DEM
Digital elevation model created in Agisoft Metashape from the dense cloud

While not really needed for our work, a byproduct of the orthorectification process is that with just a few additional processing steps, we refined the three-dimensional model of the site and created a “fly-through” animation.

Fly-through animation of three-dimensional photogrammetry model

May 3, 2022
by John Steinberg
1 Comment

Early Maps of the area around the Jeremiah Lee Mansion

In May of last year (2021) the Marblehead Museum was able to reunite the whole of the Jeremiah Lee Property when the organization purchased the neighboring Brick Kitchen at 157 Washington St.

In a May 18, 2021 article in the Marblehead Reporter by Chris Stevens, Marblehead Museum Executive Director Lauren McCormack muses that maybe one day there will be an archaeological excavation at the newly unified property.

The first phases of that archaeological excavation at the combined Lee Mansion and Brick Kitchen have begun.  In preparation for excavations in June of 2022, the staff and graduate students of the Fiske Center for Archaeological Research have been studying the site, georeferencing old maps, taking drone photos of the property, and conducting a series of geophysical surveys. 

The Mansion and Brick Kitchen were constructed in 1766.  Jeremiah Lee died in 1775, and his estate was eventually liquidated in 1788.  His widow, Martha Swett Lee, may have continued to use the house after Lee’s death.  The Lee property has been owned by the Museum since the early 20th century and served as the Marblehead Bank for most of the 19th century.  Due to the long period of institutional use (with minimal construction, demolition, and utilities), there is a potential for well-preserved 18th-century deposits on the property. 

Before we interpret the results of the geophysical surveys and plan excavations, we try to georeference every old map we can of the area in question.  There is a wonderful series of georeferenced maps on the Marblehead Historical Commission’s website.  The maps in this post have been georeferenced specifically for the Lee Mansion property.  Below is a sample of some of the many maps that we looked at, to understand the complex history of the site.

Air photo with area of investigation outlined in orange.
Air photo of area around Lee Mansion

The 2022 excavations will concentrate on the area between the Mansion and the Brick Kitchen. This area of investigation is highlighted in orange on this aerial photograph.

1850 Map of Marblehead
1850 Map of the area around Lee Mansion superimposed on air photo

While there are several earlier maps of Marblehead, the 1850 Henry McIntyre map is the first one with any details of the Lee Property.  It shows the Mansion (labeled Md. Bk), the Brick Kitchen to the east, and an upside-down “L” structure to the west that is no longer standing.  Behind and between the Mansion and the Brick Kitchen another structure is indicated.

1872 Map of Marblehead
1872 Map of the area around Lee Mansion superimposed on air photo

The 1872 map of Marblehead center, published by D.G. Beers shows the Mansion (labeled Marblehead Bank), the Brick Kitchen, and structures to the west. 

1881 Map of Marblehead
1881 Map of the area around Lee Mansion superimposed on air photo

The first really detailed map is from 1881, called the Atlas of Marblehead, published by Griffith Morgan Hopkins Jr.  On this map, structures with an x through them are stables or sheds, and brick buildings are colored pink.  Stables are indicated to the west of the Mansion and a large stable/shed is drawn at the end of the area between the Mansion and Brick Kitchen. 

1890 Map of Marblehead
1890 Map of area around Lee Mansion superimposed on air photo

An 1890 map is an extremely detailed map of the area. It is also one of the famous Sanborn fire insurance maps.  This map shows a blue dot indicating a well or well pump between the Mansion and Brick Kitchen.  The image shows the northeast kitchen addition to the Mansion.  The large shed drawn in the 1881 map (above) seems to have been replaced by a smaller storehouse.  The structures to the west of the Mansion are still indicated. 

These georeferenced maps are an important tool in understanding the development of the site and interpreting the geophysics and planning and interpreting the archaeological excavations

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