# First Photograph of the Curve of the Horizon, CurvApp overlay

On November 11, 1935 Albert W. Stevens took a specially designed infrared camera to a height of 72,395 ft over South Dakota. Albert took the first photograph of the left to right curvature of the earth. This was published in the May 1936 issue of National Geographic.

The photograph was taken with a Fairchild K-17 camera with a Kodak 304 mm f5 lens on Eastman infrared Aero film through a red filter. The film is 9″ wide (9x6) for a huge photograph.

## Exact Match of Prediction and Reality

I used my Curvature App to calculate and draw a grid that shows the exprected curvature from this height and focal length of camera. I entered the observer altitude of 72,395 ft = 22,066 m and the 35 mm equivalent focal length of 49 mm. Then I imported the image in my App to overlay it with the calculated grid. See Calculation of the 35 mm equivalent focal length.

As we can see the image matches exactly the prediction.

1935 South Dakota Original Scan → View in App
1935 South Dakota Curvature App Overlay → View in App
• Swap
• Original Scan
• Curve Calculator Overlay

Click on the images above to zoom in to high resultion and maximize the browser window. You can load the image into the Curvature App and change its transparency by clicking the image in the App (click towards left border → more transparent).

You can find an animated GIF to download, which swaps between the 2 images shown above: Image: 1935_SDakota_blink.gif

## Calculation of the 35 mm equivalent focal length

My Curvature App requires either the 35 mm equivalent focal length or the field of view of the lens used. Because the film format is not 35 mm but 9", I have to convert the focal length accordingly. I used the following equation for that:

(1)
where'
 $f_{35}$ ' =' '35 mm equivalent focal length to enter in the field Zoom f in the App $f_9$ ' =' '304 mm = focal length of the used lens $d_{35}$ ' =' '43.3 mm = film diagonal of a 35 mm film (36×24 mm) $d_9$ ' =' '274.7 mm = film diagonal of the used camera

Because I cropped the original scan slightly to exclude white borders from the slightly rotated scan, I have to zoom in the App a tic to match the cropped image. I used a value of f35 = 49 mm instead of 47.9 mm, which gives a perfect match.

The diagonal of the 9" film is calculated using the following equation. The aspect ratio is 3:2 (9"×6").

(2)
where'
 $d_9$ ' =' 'film diagonal of the used camera $w$ ' =' '9" = width of the film $h$ ' =' '6" = height of the film

## Curvature App Parameters

This are the parameters for the Curvature App, which can be copied from here and pasted in the App on the Save/Restore Tab. You can also simply click on the links below the image.

CurveApp = { "DemoText": "", "Description": "", "Height": 22066, "FocalLengthField": 49,
"Roll": 0.15, "Tilt": -5.6702762, "Pan": 0, "nLines": 45, "showModel": 1, "showGrid": 1,
"showData": true, "showTangent": true, "showEyeLevel": true, "deviceRatio": 1.5,
"viewcenterHorizon": 0, "ObjType": [ 3, 3 ], "NObjects": [ 0, 0 ], "ObjDist": [ 500, 600 ],
"ObjDeltaDist": [ 300, 300 ], "ObjSideType": [ 0, 0 ], "ObjSidePos": [ 0, 0 ],
"ObjSideVar": [ 0, 0 ], "ObjSizeType": [ 0, 0 ], "ObjSize": [ 65, 100 ], "ObjSizeVar": [ 0, 0 ],
"refractionCoeff": 0.014694115, "tempGradient": 0.00099999907, "refractionSync": 1,
"pressure": 39.587845, "temperatureC": -54.434, "refractionFactMin": 0.5, "refractionFactMax": 10000,
"rEarth": 6371000, "rFEarth": 10007543, "showTheodolite": false, "OverlayImage": "1935_SDakota.jpg",
"OverlayImageAlpha": 0.4151446, "Credit": "", "Flerspective": 0, "RefDistance": 10000,
"showRefraction": true, "showLRDrop": true }