The RRL surveys, which were described in detail in Papers I and II, were made using the Ooty Radio Telescope (ORT). ORT is a 530 m $\times$ 30 m parabolic cylinder operating at a nominal center frequency of 327 MHz (Swarup et al. 1971). The observations were made with two different angular resolutions - (a) 2 $^\circ$ $\times$ 2 $^\circ$ (low resolution mode) and (b) 2 $^\circ$ $\times$ 6 $^\prime$ (high resolution mode). The high resolution mode is obtained by using all the 22 "modules'' of the ORT, which together form a telescope of size 530 m $\times$ 30 m, and the low resolution mode is obtained by using only a single "module'' of the ORT, which effectively is a telescope of size 24 m $\times$ 30 m. The RRL transitions from principal quantum numbers n = 270, 271, 272 and 273 and $\Delta n$ = 1 were simultaneously observed using a multi-line spectrometer (Roshi 1999). The final spectrum is obtained by averaging all the four RRL transitions.

2.1 Low-resolution survey

In the low-resolution survey (Paper I), 51 positions were observed in the inner Galaxy: longitude range l = 332 $^\circ$ to 0 $^\circ$ to 89 $^\circ$ and b=0 $^\circ$ . The positions were separated in longitude by $\sim$ 2 $^\circ$ $\times$ $\sec(\delta)$ , $\delta$ being the declination. Carbon RRLs were detected from almost all directions in the longitude range l = 358 $^\circ$ $\rightarrow$ 20 $^\circ$ and also from a few positions in the longitude range l = 20 $^\circ$ to 89 $^\circ$ . In the outer Galaxy (172 $^\circ$ < l < 252 $^\circ$ ) a total of 14 positions, spaced by $\sim$ 5 $^\circ$ -7 $^\circ$ in longitude, were observed. However, no carbon RRLs were detected in this longitude range. At two specific longitudes in the inner Galaxy (l = 0 $.\!\!^\circ$ 0 and 13 $.\!\!^\circ$ 9), spectra were taken in steps of 1 $^\circ$ up to $b = \pm 4$ $^\circ$ to study the latitude extent of the carbon line emission. The observed spectra and line fit parameters were presented in Paper I.

2.2 High-resolution survey

In the high-resolution survey (Paper II), a set of seven fields which were 2 $^\circ$ wide and two fields which were 6 $^\circ$ wide in longitude were observed with a 2 $^\circ$ $\times$ 6 $^\prime$ beam. The fields are designated as Field 1 to 9 and are centered at l = 348 $.\!\!^\circ$ 0 (2 $^\circ$ wide), 3 $.\!\!^\circ$ 4 (6 $^\circ$ ), 13 $.\!\!^\circ$ 9 (2 $^\circ$ ), 25 $.\!\!^\circ$ 2 (2 $^\circ$ ), 27 $.\!\!^\circ$ 5 (2 $^\circ$ ), 36 $.\!\!^\circ$ 3 (6 $^\circ$ ), 45 $.\!\!^\circ$ 5 (2 $^\circ$ ), 56 $.\!\!^\circ$ 9 (2 $^\circ$ ) and 66 $.\!\!^\circ$ 2 (2 $^\circ$ ) respectively. The ORT is an equatorially mounted telescope and the beam size is 2 $^\circ$ along right ascension. The orientation of the beam with respect to galactic co-ordinates, therefore, changes as a function of galactic longitude. Carbon RRLs were detected toward several positions within the fields with l < 40 $^\circ$ , whereas no lines were detected within the fields in the longitude range l = 40 $^\circ$ to 85 $^\circ$ . The galactic coordinates of the positions where carbon lines are detected and the parameters estimated from Gaussian fits to the line profiles are given in Table 1. Each spectrum was inspected by eye and the presence of a carbon line was determined. If narrow ( $\sim$ 1-2 channels) spectral features were present in addition to the carbon line feature, we regarded the detection as tentative. The narrow spurious features were either due to residual radio frequency interference or "bad'' spectral channel values, which were inferred from the channel weights as discussed in Paper I. However, if the width of the carbon line was several times ( $\sim$ 10) larger than any spurious narrow features then we regarded them as real. Since the peak line intensity to the rms noise in the spectra is about 3 to 4, care has been taken in fitting Gaussian components to the line profile. A second Gaussian component was fitted to only those spectra where the residuals left after removing a single Gaussian component were inconsistent with the noise in rest of the spectrum. The details of the high-resolution survey and the observed spectra were presented in Paper II.

Table 1: Summary of the carbon RRL observations from the high resolution survey.
l b $T_{\rm L}/T_{{\rm sys}}$ ¹ $\Delta V$ $V_{\rm {LSR}}$ $V_{{\rm res}}$ ² rms ³ $t_{{\rm int}}$

$^\circ$ $^\circ$ $\times$ 10³ km s^-1 km s^-1 km s^-1 $\times$ 10³ hrs

Field 2a

0.52 +0.03 0.64(0.08) 22.4(3.2) 1.7(1.4) 1.8 0.20 12.8

0.67 -0.00 0.66(0.18) 4.3(1.3) 6.7(0.6) 1.8 0.19 11.2

0.37(0.15) 5.8(2.7) -11.1(1.2) 1.8 0.19 11.2

0.75 +0.05 0.54(0.11) 9.8(2.3) 18.9(1.0) 3.4 0.13 11.6

0.41(0.09) 14.8(3.7) -7.2(1.6) 3.4 0.13 11.6

0.84 +0.10 0.69(0.12) 15.0(3.0) 6.3(1.3) 3.4 0.18 9.4

0.92 +0.16 0.83(0.22) 4.0(1.2) 6.5(0.5) 1.8 0.23 12.6

Field 2b (G2.3+0.0)

1.21 +0.07 0.32(0.1) ⁴ 11.1(4.2) 2.6(1.7) 3.4 0.13 10.3

0.33(0.11) ⁴ 9.2(3.6) -17.8(1.5) 3.4 0.13 10.3

1.29 +0.13 0.58(0.08) 25.5(4.2) 3.6(1.8) 2.1 0.21 9.9

1.38 +0.18 0.55(0.08) 30.1(5.3) 1.6(2.2) 3.4 0.17 10.1

1.83 +0.20 0.63(0.15) 7.3(2.0) 1.9(0.9) 2.1 0.20 11.3

2.29 +0.21 0.47(0.09) 14.3(3.1) 10.3(1.3) 2.1 0.17 14.9

2.54 -0.03 0.53(0.13) 14.3(4.0) 8.9(1.7) 2.1 0.24 8.1

2.63 +0.02 0.55(0.12) 10.3(2.6) 4.0(1.1) 2.1 0.19 11.2

2.78 -0.03 0.34(0.08) 18.0(4.6) 11.1(1.9) 2.1 0.16 11.2

2.86 +0.02 0.32(0.07) 28.7(7.9) 10.6(3.3) 2.1 0.20 12.5

3.01 -0.03 0.44(0.08) 26.3(5.3) 9.3(2.2) 2.1 0.19 11.2

3.09 +0.02 0.49(0.12) 12.8(3.6) 5.8(1.5) 2.1 0.21 11.7

3.33 +0.02 0.46(0.1) 14.0(3.7) 6.9(1.5) 2.1 0.19 11.5

Field 2c (G4.7+0.0)

3.56 +0.02 0.33(0.08) 19.5(5.3) 10.0(2.2) 3.4 0.13 10.4

3.79 +0.02 0.45(0.12) 11.8(3.6) 9.8(1.5) 3.4 0.16 11.6

3.94 -0.03 0.66(0.15) 8.8(2.3) 11.6(1.0) 2.1 0.22 8.4

4.26 +0.02 0.72(0.15) 7.8(1.9) 5.8(0.8) 2.1 0.21 12.2

4.49 +0.02 0.40(0.12) ⁴ 14.4(5.0) 9.2(2.1) 3.4 0.17 11.6

4.64 -0.03 0.27(0.08) ⁴ 22.8(7.9) 8.0(3.3) 3.4 0.15 9.6

4.72 +0.02 0.39(0.09) 28.2(7.5) 10.6(3.2) 3.4 0.18 11.5

4.87 -0.03 0.79(0.18) ⁴ 7.0(1.9) 11.9(0.8) 1.8 0.25 8.3

0.75(0.22) ⁴ 4.9(1.6) -35.5(0.7) 1.8 0.25 8.3

4.95 +0.02 0.64(0.14) 11.4(2.8) 9.0(1.2) 2.1 0.23 8.5

5.19 +0.02 0.78(0.15) 8.1(1.8) 8.4(0.8) 1.8 0.23 9.0

5.33 -0.03 0.54(0.07) 22.6(3.6) 12.8(1.5) 2.1 0.17 10.2

5.42 +0.02 0.49(0.09) 20.1(4.0) 9.1(1.7) 2.1 0.19 10.6

5.56 -0.03 0.71(0.19) ⁴ 4.3(1.4) 7.5(0.6) 2.1 0.20 10.0

5.65 +0.02 0.79(0.15) 7.1(1.5) 5.6(0.6) 2.1 0.19 12.3

5.80 -0.03 0.80(0.15) 9.1(1.9) 5.2(0.8) 2.1 0.22 7.4

0.72(0.19) 5.5(1.7) 18.4(0.8) 2.1 0.22 7.4

5.88 +0.02 0.51(0.13) 14.0(4.0) 13.7(1.7) 2.1 0.23 8.6

6.02 -0.02 0.43(0.14) ⁴ 20.3(7.7) 6.0(3.2) 3.4 0.24 4.2

6.25 -0.02 0.67(0.14) ⁴ 16.0(3.9) 0.6(1.6) 2.1 0.27 5.6

6.72 -0.02 0.53(0.12) 9.7(2.6) 6.1(1.1) 2.1 0.19 10.0

6.80 +0.03 0.58(0.12) 8.6(2.0) 7.7(0.9) 2.1 0.17 11.4

**Table 1:** Summary of the carbon RRL observations from the high resolution survey.
l	b	$T_{\rm L}/T_{{\rm sys}}$ ¹	$\Delta V$	$V_{\rm {LSR}}$	$V_{{\rm res}}$ ²	rms ³	$t_{{\rm int}}$
$^\circ$	$^\circ$	$\times$ 10³	km s^-1	km s^-1	km s^-1	$\times$ 10³	hrs
Field 2a
0.52	+0.03	0.64(0.08)	22.4(3.2)	1.7(1.4)	1.8	0.20	12.8
0.67	-0.00	0.66(0.18)	4.3(1.3)	6.7(0.6)	1.8	0.19	11.2
		0.37(0.15)	5.8(2.7)	-11.1(1.2)	1.8	0.19	11.2
0.75	+0.05	0.54(0.11)	9.8(2.3)	18.9(1.0)	3.4	0.13	11.6
		0.41(0.09)	14.8(3.7)	-7.2(1.6)	3.4	0.13	11.6
0.84	+0.10	0.69(0.12)	15.0(3.0)	6.3(1.3)	3.4	0.18	9.4
0.92	+0.16	0.83(0.22)	4.0(1.2)	6.5(0.5)	1.8	0.23	12.6
Field 2b (G2.3+0.0)
1.21	+0.07	0.32(0.1) ⁴	11.1(4.2)	2.6(1.7)	3.4	0.13	10.3
		0.33(0.11) ⁴	9.2(3.6)	-17.8(1.5)	3.4	0.13	10.3
1.29	+0.13	0.58(0.08)	25.5(4.2)	3.6(1.8)	2.1	0.21	9.9
1.38	+0.18	0.55(0.08)	30.1(5.3)	1.6(2.2)	3.4	0.17	10.1
1.83	+0.20	0.63(0.15)	7.3(2.0)	1.9(0.9)	2.1	0.20	11.3
2.29	+0.21	0.47(0.09)	14.3(3.1)	10.3(1.3)	2.1	0.17	14.9
2.54	-0.03	0.53(0.13)	14.3(4.0)	8.9(1.7)	2.1	0.24	8.1
2.63	+0.02	0.55(0.12)	10.3(2.6)	4.0(1.1)	2.1	0.19	11.2
2.78	-0.03	0.34(0.08)	18.0(4.6)	11.1(1.9)	2.1	0.16	11.2
2.86	+0.02	0.32(0.07)	28.7(7.9)	10.6(3.3)	2.1	0.20	12.5
3.01	-0.03	0.44(0.08)	26.3(5.3)	9.3(2.2)	2.1	0.19	11.2
3.09	+0.02	0.49(0.12)	12.8(3.6)	5.8(1.5)	2.1	0.21	11.7
3.33	+0.02	0.46(0.1)	14.0(3.7)	6.9(1.5)	2.1	0.19	11.5
Field 2c (G4.7+0.0)
3.56	+0.02	0.33(0.08)	19.5(5.3)	10.0(2.2)	3.4	0.13	10.4
3.79	+0.02	0.45(0.12)	11.8(3.6)	9.8(1.5)	3.4	0.16	11.6
3.94	-0.03	0.66(0.15)	8.8(2.3)	11.6(1.0)	2.1	0.22	8.4
4.26	+0.02	0.72(0.15)	7.8(1.9)	5.8(0.8)	2.1	0.21	12.2
4.49	+0.02	0.40(0.12) ⁴	14.4(5.0)	9.2(2.1)	3.4	0.17	11.6
4.64	-0.03	0.27(0.08) ⁴	22.8(7.9)	8.0(3.3)	3.4	0.15	9.6
4.72	+0.02	0.39(0.09)	28.2(7.5)	10.6(3.2)	3.4	0.18	11.5
4.87	-0.03	0.79(0.18) ⁴	7.0(1.9)	11.9(0.8)	1.8	0.25	8.3
		0.75(0.22) ⁴	4.9(1.6)	-35.5(0.7)	1.8	0.25	8.3
4.95	+0.02	0.64(0.14)	11.4(2.8)	9.0(1.2)	2.1	0.23	8.5
5.19	+0.02	0.78(0.15)	8.1(1.8)	8.4(0.8)	1.8	0.23	9.0
5.33	-0.03	0.54(0.07)	22.6(3.6)	12.8(1.5)	2.1	0.17	10.2
5.42	+0.02	0.49(0.09)	20.1(4.0)	9.1(1.7)	2.1	0.19	10.6
5.56	-0.03	0.71(0.19) ⁴	4.3(1.4)	7.5(0.6)	2.1	0.20	10.0
5.65	+0.02	0.79(0.15)	7.1(1.5)	5.6(0.6)	2.1	0.19	12.3
5.80	-0.03	0.80(0.15)	9.1(1.9)	5.2(0.8)	2.1	0.22	7.4
		0.72(0.19)	5.5(1.7)	18.4(0.8)	2.1	0.22	7.4
5.88	+0.02	0.51(0.13)	14.0(4.0)	13.7(1.7)	2.1	0.23	8.6
6.02	-0.02	0.43(0.14) ⁴	20.3(7.7)	6.0(3.2)	3.4	0.24	4.2
6.25	-0.02	0.67(0.14) ⁴	16.0(3.9)	0.6(1.6)	2.1	0.27	5.6
6.72	-0.02	0.53(0.12)	9.7(2.6)	6.1(1.1)	2.1	0.19	10.0
6.80	+0.03	0.58(0.12)	8.6(2.0)	7.7(0.9)	2.1	0.17	11.4

**Table 1:** continued.
Field 3 (G13.9+0.0)
13.04	-0.46	0.46(0.07)	36.0(6.6)	40.6(2.8)	2.1	0.21	7.9
13.13	-0.41	0.46(0.14)	7.0(2.5)	52.1(1.0)	2.1	0.18	10.6
		0.89(0.11)	10.8(1.6)	35.2(0.7)	2.1	0.18	10.6
		0.64(0.11)	12.5(2.4)	15.7(1.0)	2.1	0.18	10.6
13.22	-0.36	0.41(0.08)	37.2(8.1)	39.9(3.4)	1.8	0.24	8.2
		0.61(0.16)	8.0(2.5)	18.4(1.1)	1.8	0.24	8.2
13.30	-0.31	0.51(0.1)	24.8(5.8)	42.3(2.4)	1.8	0.26	8.4
		0.99(0.17)	8.6(1.7)	19.8(0.7)	1.8	0.26	8.4
13.39	-0.26	0.44(0.12)	14.4(4.6)	11.8(2.0)	2.1	0.23	8.4
		0.33(0.09)	29.8(8.8)	39.5(3.7)	2.1	0.23	8.4
13.48	-0.22	0.38(0.11)	28.6(9.7)	44.0(4.1)	3.4	0.23	11.2
		0.50(0.15)	16.5(5.6)	14.5(2.4)	3.4	0.23	11.2
13.57	-0.17	0.77(0.18)	5.4(1.5)	19.3(0.6)	2.1	0.21	9.3
		0.43(0.07)	38.9(7.0)	33.1(3.0)	2.1	0.21	9.3
13.65	-0.12	0.41(0.09)	20.8(5.4)	24.8(2.3)	2.1	0.20	9.1
13.74	-0.07	0.57(0.17) ⁴	9.0(3.2)	18.0(1.3)	2.1	0.25	8.0
13.83	-0.02	0.49(0.14) ⁴	10.1(3.3)	44.5(1.4)	2.1	0.21	8.8
		0.66(0.19) ⁴	5.2(1.8)	18.7(0.7)	2.1	0.21	8.8
13.92	+0.03	0.84(0.21) ⁴	6.5(1.9)	19.2(0.8)	2.1	0.26	6.3
14.09	+0.12	0.45(0.05)	47.3(6.3)	34.3(2.7)	2.1	0.18	10.0
14.18	+0.17	0.35(0.09)	19.1(5.7)	39.8(2.4)	2.1	0.19	10.9
		0.61(0.13)	8.6(2.2)	19.6(0.9)	2.1	0.19	10.9
14.36	+0.26	0.58(0.06)	29.3(3.6)	43.6(1.5)	2.1	0.16	11.7
		0.57(0.1)	11.3(2.3)	18.7(1.0)	2.1	0.16	11.7
14.44	+0.31	0.52(0.08)	27.1(5.1)	19.6(2.1)	2.1	0.21	7.5
14.62	+0.41	0.52(0.14)	9.9(3.1)	26.9(1.3)	2.1	0.22	10.1
14.71	+0.46	0.35(0.1)	12.8(4.4)	51.5(1.9)	2.1	0.18	11.8
		0.43(0.08)	20.0(4.5)	23.0(1.9)	2.1	0.18	11.8
Field 5 (G27.5+0.0)
27.06	-0.20	0.41(0.09)	19.4(5.1)	58.2(2.2)	3.4	0.16	8.0
28.04	+0.31	0.66(0.15)	6.6(1.7)	79.5(0.7)	1.8	0.20	15.0
28.13	+0.35	0.34(0.07)	20.7(5.1)	73.2(2.2)	3.4	0.13	18.6
Field 6a (G34.2+0.0)
34.18	-0.02	0.63(0.15) ⁴	8.1(2.3)	44.4(1.0)	2.1	0.22	9.7
34.27	+0.03	0.36(0.08)	15.2(3.9)	49.2(1.6)	2.1	0.15	13.1
34.85	-0.02	0.55(0.08)	31.3(5.6)	43.8(2.3)	2.1	0.23	8.9
34.94	+0.03	0.45(0.1)	18.1(4.7)	41.0(2.0)	2.1	0.21	11.0
35.08	-0.02	0.38(0.1)	17.4(5.0)	51.6(2.1)	3.4	0.15	8.9
35.17	+0.03	0.51(0.08)	26.6(5.1)	50.2(2.2)	3.4	0.17	11.9
35.31	-0.02	0.23(0.07)	19.1(7.0)	66.4(2.9)	7.6	0.82	11.1
Field 6b (G36.5+0.0)
35.76	-0.02	0.56(0.10)	12.6(2.5)	51.5(1.0)	2.1	0.17	10.7
36.21	-0.02	0.43(0.07)	30.3(5.5)	52.7(2.3)	2.1	0.18	10.5

2.3 Line width

If we assume that the physical properties of the carbon line emitting gas observed in the galactic disk are similar to those derived for the gas toward Cas A (e.g. $T_{\rm e} = 75$ K, $n_{\rm e}=0.1$ cm^-3, from Payne et al. 1994) and the galactic background radiation temperature =700 K (from Paper I), then the total contribution from pressure, radiation and Doppler broadening at 325 MHz would be a negligible $\sim$ 0.6 km s^-1. Comparing this with the observed line widths in the high-resolution data ranging from 4 to 48 km s^-1 with a median value of 14 km s^-1 (Fig. 1), it is clear that the cause of line broadening lies elsewhere. Blending of carbon line features from different line forming regions within the coarse survey beam and turbulent motions within the cloud are likely the cause of the broad lines. To confirm this we have examined the data toward a few directions in the galactic plane in more detail and the results (see Sect. 6 for details) support our conclusion. Moreover, the median line width obtained from the low-resolution survey data is 17 km s^-1 (Paper I), which is somewhat larger than the value estimated from the higher resolution data ( $\sim$ 14 km s^-1). The larger median value is likely a result of line blending because of the relatively larger beam width of the low-resolution survey. The line widths of carbon lines observed in the survey are typically a factor of 2 to 5 larger than the typical line width of carbon lines observed at frequencies >1 GHz from "classical'' C II regions (e.g. Roelfsema & Goss 1992). Since the lines at low and high frequencies are believed to arise in distinct regions of the ISM, the difference is not surprising.

2 Summary of observations and basic results

2.1 Low-resolution survey

2.2 High-resolution survey

2.3 Line width