Algorithms

Submenu on Analyze menu

Vertical earth curvature (requires open DEM)
Horizontal earth curvature (requires open DEM)

Arc second spacing conversion factors: this is the factor needed to sort of get GCS DEMs to work correctly in some software. You can use only one factor, and must pick if you want the X direction, the Y direction, or the average. Shows the number of meters in a degree and an arc second. You can get a graph from the database created.

LAT	M_DEG_Y	M_DEG_X	M_DEG_AVG	M_SEC_Y	M_SEC_X	M_SEC_AVG
85	111685.375	9734.4	60709.887	31.024	2.704	16.864
84	111681.615	11674.624	61678.12	31.023	3.243	17.133
83	111677.184	13611.221	62644.202	31.021	3.781	17.401
82	111672.085	15543.589	63607.837	31.02	4.318	17.669
81	111666.326	17471.131	64568.728	31.018	4.853	17.936
80	111659.913	19393.247	65526.58	31.017	5.387	18.202

Output

Table for every 1 degree of latitude (same results in both hemispheres)
Can get graphs in MICRODEM or Excel, and you can filter the results as desired

Gives

Meters per degree in the x, and y directions, and the average
Meters per second in the x, and y directions, and the average
These can be used, not very effectivley but it's they best they can do, in software that cannot do Slope algorithms for arc second DEMs

Arc second rectangular pixel approximation

LAT	SEC	SIDE_M	BASE_M	TOP_M	ERROR_PC	RECT
85	1	31.0237	2.704	2.7038	0.00554088	0.0872
85	3	93.0712	8.1119	8.1105	0.01662265	0.0871
85	30	930.7118	81.121	80.9862	0.16623594	0.087
85	60	1861.4239	162.242	161.7026	0.33247403	0.0869
85	450	13960.7019	1216.8151	1186.4706	2.49376366	0.085
85	3600	111687.0008	9734.4	7791.152	19.96268876	0.0698

Output

Table for every 5 degrees of latitude (same results in both hemispheres), for 5 spacings in arc seconds
Can get graphs in MICRODEM or Excel, and you can filter the results as desired

Applicable to a quadrangle map, or the pixels in a geographic grid.

1" and 3" are the pixel sizes of the Key Global Regional DEMs
60" is the pixel size of the 1' ETOPO1
450" is a 7.5' quadrangle
3600" is a one degree tile of the Key Global Regional DEMs

Gives

the size in meters of the side (same east and west), top, and bottom each each quad/pixel
ERROR_PC Gives the percent difference between the top and bottom. This will be a max at the pole, and zero at the equator
RECT gives the ratio of TOP/BOTTOM, on how rectangular the pixels are

UTM spacing and grid-true angle

LAT	W_SPACE	W_ROTATE	MID_SPACE	MID_ROTATE	E_SPACE	E_ROTATE
80	30.011	357.05	30.012	0	30.011	2.95
79	30.011	357.06	30.012	0	30.011	2.94
78	30.01	357.07	30.012	0	30.01	2.93
77	30.01	357.08	30.012	0	30.01	2.92
76	30.01	357.09	30.012	0	30.01	2.91
75	30.009	357.1	30.012	0	30.009	2.9

5	29.971	359.74	30.012	0	29.971	0.26
4	29.971	359.79	30.012	0	29.971	0.21
3	29.971	359.84	30.012	0	29.971	0.16
2	29.971	359.9	30.012	0	29.971	0.1
1	29.971	359.95	30.012	0	29.971	0.05
0	29.971	0	30.012	0	29.971	0

For every one degree of latitude for a UTM zone and a 30 m DEM, gives

Spacing in meters for the western edge, center, and and eastern edge of the zone. These are true meters, and the projected metgers are a constant 30 m
The grid north to trun north rotation angle at the same three locations

GDAL slopes arc second DEMs (requires open DEM): creates 6 maps using the Zevenbergen and Thorne algorithms, with three spacings: the x,y, and average. None give correct answers; compare with the slope maps from MICRODEM.

Last revision 7/16/2021