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Using GIS to Estimate Sites of Potential Vernal Pool Occurrence in Barnstable, Massachusetts

This project examined how GIS could be used to identify potential locations of vernal pools by combining and analyzing spatial data. By using data associated with previously certified vernal pools, the characteristics of the topography for these pools was used as a benchmark to determine potential other sites. The primary question answered by this project was if the trends from current vernal pools could be extrapolated to locate potential other vernal pools. Spatial analyst tools such as near, convert, reclassify, and calculate were used to accomplish this. The end result was a map of Barnstable showing the relative likelihood of finding a vernal pool based on the characteristics it shared with other vernal pool locations.

All of the data for this project was obtained from the MassGIS website. The following layers were utilized:

All of the existing layers were cropped to the area of interest, the town of Barnstable, by using the select by location tool and creating a new layer. The land use layer was reclassified by dividing the land use into 3 categories: most open space, some open space, little to no open space. The following is the metadata table describing each land use code and its definition:

CODE

 

ABBREV

 

CATEGORY

 

DEFINITION

1

 

AC

 

Cropland

 

Intensive agriculture

2

 

AP

 

Pasture

 

Extensive agriculture

3

 

F

 

Forest

 

Forest

4

 

FW

 

Wetland

 

Nonforested freshwater wetland

5

 

M

 

Mining

 

Sand; gravel & rock

6

 

O

 

Open Land

 

Abandoned agriculture; power lines; areas of no vegetation

7

 

RP

 

Participation Recreation

 

Golf; tennis; Playgrounds; skiing

8

 

RS

 

Spectator Recreation

 

Stadiums; racetracks; Fairgrounds; drive-ins

9

 

RW

 

Water Based Recreation

 

Beaches; marinas; Swimming pools

10

 

R0

 

Residential

 

Multi-family

11

 

R1

 

Residential

 

Smaller than 1/4 acre lots

12

 

R2

 

Residential

 

1/4 - 1/2 acre lots

13

 

R3

 

Residential

 

Larger than 1/2 acre lots

14

 

SW

 

Salt Wetland

 

Salt marsh

15

 

UC

 

Commercial

 

General urban; shopping center

16

 

UI

 

Industrial

 

Light & heavy industry

17

 

UO

 

Urban Open

 

Parks; cemeteries; public & institutional greenspace; also vacant undeveloped land

18

 

UT

 

Transportation

 

Airports; docks; divided highway; freight; storage; railroads

19

 

UW

 

Waste Disposal

 

Landfills; sewage lagoons

20

 

W

 

Water

 

Fresh water; coastal embayment

21

 

WP

 

Woody Perennial

 

Orchard; nursery; cranberry bog

22

 

-

 

No Change

 

Code used by MassGIS only during quality checking

CODE

 

ABBREV

 

CATEGORY

23

 

CB

 

Cranberry bog (part of #21)

24

 

PL

 

Powerlines (part of #6)

25

 

RSB

 

Salwater sandy beach (part of #9; no longer used)

26

 

RG

 

Golf (part of #7)

27

 

TSM

 

Tidal salt marshes (part of #14; no longer used)

28

 

ISM

 

Irregulary flooded salt marshes (part of #14; no longer used)

29

 

RM

 

Marina (part of #9)

30

 

-

 

New ocean (areas of accretion; part of #20)

31

 

UP

 

Urban public (part of #17)

32

 

TF

 

Transportation facilities (part of #18)

33

 

H

 

Heath (part of #17)

34

 

CM

 

Cemeteries (part of #17)

35

 

OR

 

Orchard (part of #21)

36

 

N

 

Nursery (part of #21)

37

 

-


Forested wetland (part of #3; no longer used


The following is the reclassification categories of each land use code. Codes not included did not occur in the data for Barnstable. 

Most open space-3, 6, 14, 21, 23, 35

Some open space-1, 2, 4, 7, 13, 17, 19, 24, 26, 34, 36

Little to no open space- 5, 8, 9, 10, 11, 12, 15, 16, 18, 20, 29, 30, 31, 32

Using the extract values to points function, it was determined that 22 of the 26 vernal pools occurred in areas listed as "most open space."

Using the near function, the distance from each vernal pool to the nearest source of water was determined, after converting the hydro data to a raster dataset. It was found that:

Average distance to water = 114.01 m

Min= 1.75 m

Max = 328.81 m 

8 pools 150-300 meters away from water

10 pools 50-150 meters away

4 pools 10-50 meters away

4 pools 1-10 meters away

Most of the pools were found within a close distance to water sources, indicating that searches should be concentrated within a few meters of a water source.

Using the extract values to points function,  the slope of the soil that each vernal pool was located on was determined. It was found that:

  •       2 at 0%-  water or urban land
  •       5 at 1 15-25% Slope
  •       8 at 2 8-15% Slope
  •       10 at 3 3-8% Slope
  •       1 at 4 0-3% Slope 

The majority of the pools were found at a slope of 3-15%, indicating that the pools are more likely to be found on areas of some slope, potentially allowing natural valleys to form and the pools to fill.

Each of these datasets were then reclassified from 1 to 3, 3 indicating the feature most likely to be found at a vernal pool. 0 was given to water, areas where no vernal pools could occur. The data was then calculated, weighting the layer based on its correlation to vernal pool location. Since land use correlated most directly to vernal pool occurrence, it was weighted the heaviest.

Land Use x .50

Soil Slope x .25

Distance to Hydro x .25 

This created a layer with values from 0 to 3, 3 indicating the site was very likely to contain a vernal pool.

The largest problem encountered with this project was the program crashing when the reclassify or convert to raster function was utilized for complex data sets.  This was remedied by ensuring there was enough space in the drive to save the resulting layer, and utilizing a large amount of patience. Some problems were also encountered trying to insert the maps onto the poster and having them seem blurry. This was prevented by saving the map as a jpeg and inserting it into Publisher instead of using snag-it.

In conclusion, this project created a map that answered the main question proposed at the start and created a useful tool to direct future vernal pool searches. By overlaying a road dataset (not included to reduce clutter on the map), searches could be directed very precisely to areas that share similar characteristics to previously certified vernal pools. In the future, this could be done with more datasets and more correlating data, such as elevation and distance from major roadways. It could also be done in other towns to determine if the criteria hold true for other sites. This map is just the start of applying GIS to locate vernal pools. 

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